Celery does not consider authMechanism on mongodb backend URLs

# Checklist


- [x] I have read the relevant section in the
 [contribution guide](http://docs.celeryproject.org/en/latest/contributing.html#other-bugs)
 on reporting bugs.
- [x] I have checked the [issues list](https://github.com/celery/celery/issues?q=is%3Aissue+label%3A%22Issue+Type%3A+Bug+Report%22+-label%3A%22Category%3A+Documentation%22)
 for similar or identical bug reports.
- [x] I have checked the [pull requests list](https://github.com/celery/celery/pulls?q=is%3Apr+label%3A%22PR+Type%3A+Bugfix%22+-label%3A%22Category%3A+Documentation%22)
 for existing proposed fixes.
- [x] I have checked the [commit log](https://github.com/celery/celery/commits/master)
 to find out if the bug was already fixed in the master branch.
- [x] I have included all related issues and possible duplicate issues
 in this issue (If there are none, check this box anyway).

## Mandatory Debugging Information

- [x] I have included the output of ``celery -A proj report`` in the issue.
 (if you are not able to do this, then at least specify the Celery
 version affected).
- [x] I have verified that the issue exists against the `master` branch of Celery.
- [x] I have included the contents of ``pip freeze`` in the issue.
- [x] I have included all the versions of all the external dependencies required
 to reproduce this bug.

## Optional Debugging Information

- [ ] I have tried reproducing the issue on more than one Python version
 and/or implementation.
- [ ] I have tried reproducing the issue on more than one message broker and/or
 result backend.
- [ ] I have tried reproducing the issue on more than one version of the message
 broker and/or result backend.
- [ ] I have tried reproducing the issue on more than one operating system.
- [ ] I have tried reproducing the issue on more than one workers pool.
- [ ] I have tried reproducing the issue with autoscaling, retries,
 ETA/Countdown & rate limits disabled.
- [ ] I have tried reproducing the issue after downgrading
 and/or upgrading Celery and its dependencies.

## Related Issues and Possible Duplicates


#### Related Issues

- #4454 

#### Possible Duplicates

- None

## Environment & Settings

**Celery version**: 4.3.0 with fixes in PR #5527 

<details>
<summary><code>celery report</code> Output:</summary>


```
software -> celery:4.3.0 (rhubarb) kombu:4.6.4 py:3.7.4
 billiard:3.6.1.0 py-amqp:2.5.1
platform -> system:Windows arch:64bit, WindowsPE
 kernel version:10 imp:CPython
loader -> celery.loaders.default.Loader
settings -> transport:amqp results:disabled
```


</details>

# Steps to Reproduce
1. Give Celery a Backend URL pointing to a MongoDB like below:
mongodb://admin:admin@192.168.0.1/task?authSource=admin&authMechanism=SCRAM-SHA-256
2. Start Celery worker.
3. Send any task.

## Required Dependencies

* **Minimal Python Version**: 3.0
* **Minimal Celery Version**: 3.0
* **Minimal Kombu Version**: Unknown
* **Minimal Broker Version**: Unknown
* **Minimal Result Backend Version**: MongoDB 4.0
* **Minimal OS and/or Kernel Version**: N/A
* **Minimal Broker Client Version**: Unknown
* **Minimal Result Backend Client Version**: Unknown

### Python Packages

<details>
<summary><code>pip freeze</code> Output:</summary>


```
amqp==2.5.1
asn1crypto==0.24.0
astroid==2.2.5
bcrypt==3.1.7
billiard==3.6.1.0
celery==4.3.0
cffi==1.12.3
Click==7.0
colorama==0.4.1
cryptography==2.7
irectory=backend
Flask==1.1.1
importlib-metadata==0.19
isort==4.3.21
itsdangerous==1.1.0
Jinja2==2.10.1
kombu==4.6.4
lazy-object-proxy==1.4.2
MarkupSafe==1.1.1
mccabe==0.6.1
more-itertools==7.2.0
paramiko==2.6.0
pycparser==2.19
pylint==2.3.1
pymodm==0.4.1
pymongo==3.9.0
PyNaCl==1.3.0
pytz==2019.2
six==1.12.0
typed-ast==1.4.0
vine==1.3.0
Werkzeug==0.15.5
wrapt==1.11.2
zipp==0.6.0
```


</details>

### Other Dependencies

<details>
<summary>MongoDB:</summary>

Version: >= 3.0.0

</details>

## Minimally Reproducible Test Case


<details>


```python
```


</details>

# Expected Behavior

Support all the authentication options listed at [authentication-options](https://docs.mongodb.com/manual/reference/connection-string/#authentication-options), including _authSource_, _authMechanism_, _authMechanismProperties_ and _gssapiServiceName_.

# Actual Behavior

An error occurred as below.
```python
Traceback (most recent call last):
 File "D:\GitHub\org\dev\.venv\lib\site-packages\kombu\utils\objects.py", line 42, in __get__
 return obj.__dict__[self.__name__]
KeyError: 'collection'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "D:\GitHub\org\dev\.venv\lib\site-packages\kombu\utils\objects.py", line 42, in __get__
 return obj.__dict__[self.__name__]
KeyError: 'database'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "c:\Users\user\.vscode\extensions\ms-python.python-2019.9.34911\pythonFiles\ptvsd_launcher.py", line 43, in <module>
 main(ptvsdArgs)
 File "c:\Users\user\.vscode\extensions\ms-python.python-2019.9.34911\pythonFiles\lib\python\ptvsd\__main__.py", line 432, in main
 run()
 File "c:\Users\user\.vscode\extensions\ms-python.python-2019.9.34911\pythonFiles\lib\python\ptvsd\__main__.py", line 316, in run_file
 runpy.run_path(target, run_name='__main__')
 File "C:\Program Files\Python37\lib\runpy.py", line 263, in run_path
 pkg_name=pkg_name, script_name=fname)
 File "C:\Program Files\Python37\lib\runpy.py", line 96, in _run_module_code
 mod_name, mod_spec, pkg_name, script_name)
 File "C:\Program Files\Python37\lib\runpy.py", line 85, in _run_code
 exec(code, run_globals)
 File "d:\GitHub\org\dev\backend\tests\workertest.py", line 22, in <module>
 print('Task finished? ', result.ready())
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\result.py", line 313, in ready
 return self.state in self.backend.READY_STATES
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\result.py", line 473, in state
 return self._get_task_meta()['status']
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\result.py", line 412, in _get_task_meta
 return self._maybe_set_cache(self.backend.get_task_meta(self.id))
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\base.py", line 386, in get_task_meta
 meta = self._get_task_meta_for(task_id)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\mongodb.py", line 206, in _get_task_meta_for
 obj = self.collection.find_one({'_id': task_id})
 File "D:\GitHub\org\dev\.venv\lib\site-packages\kombu\utils\objects.py", line 44, in __get__
 value = obj.__dict__[self.__name__] = self.__get(obj)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\mongodb.py", line 293, in collection
 collection = self.database[self.taskmeta_collection]
 File "D:\GitHub\org\dev\.venv\lib\site-packages\kombu\utils\objects.py", line 44, in __get__
 value = obj.__dict__[self.__name__] = self.__get(obj)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\mongodb.py", line 288, in database
 return self._get_database()
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\mongodb.py", line 271, in _get_database
 conn = self._get_connection()
 File "D:\GitHub\org\dev\.venv\lib\site-packages\celery\backends\mongodb.py", line 161, in _get_connection
 self._connection = MongoClient(**conf)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\pymongo\mongo_client.py", line 668, in __init__
 username, password, dbase, opts)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\pymongo\client_options.py", line 151, in __init__
 username, password, database, options)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\pymongo\client_options.py", line 39, in _parse_credentials
 mechanism, source, username, password, options, database)
 File "D:\GitHub\org\dev\.venv\lib\site-packages\pymongo\auth.py", line 107, in _build_credentials_tuple
 raise ConfigurationError("%s requires a username." % (mech,))
pymongo.errors.ConfigurationError: SCRAM-SHA-256 requires a username.
```

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Celery does not consider authMechanism on mongodb backend URLs #5734

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Celery does not consider authMechanism on mongodb backend URLs #5734

Description

Checklist

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Optional Debugging Information

Related Issues and Possible Duplicates

Related Issues

Possible Duplicates

Environment & Settings

Steps to Reproduce

Required Dependencies

Python Packages

Other Dependencies

Minimally Reproducible Test Case

Expected Behavior

Actual Behavior

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