Description
Running the test case: Fib with topology: t0-64-32, the file:/root/fib_info.txt is always empty.
This empty fib_info.txt caused fail , because it read this file to get ip range(_ipv4_lpm_dict or _ipv6_lpm_dict) in fib.py.

Steps to reproduce the issue:

1. From shared-fib.yml, we know the fib_info.txt is created from the templates: fib.j2. as below:

• name: Generate route-port map information
  template: src=roles/test/templates/fib.j2
  dest=/root/fib_info.txt
  delegate_to: "{{ptf_host}}"

2. there is no 't0-64-32' defined as testbed_type in fib.j2 , like the following:
   {% if testbed_type == 't1' %}

Describe the results you received:
The more detail error messages as blow:
"=============================================================",
"ERROR: fib_test.FibTest",
"----------------------------------------------------------------------",
"Traceback (most recent call last):",
" File "ptftests/fib_test.py", line 286, in runTest",
" self.check_ip_range()",
" File "ptftests/fib_test.py", line 121, in check_ip_range",
" exp_port_list = self.fib[ip_range.get_first_ip()].get_next_hop_list()",
" File "ptftests/fib.py", line 71, in getitem",
" return self._ipv4_lpm_dict[str(ip)]",
" File "ptftests/lpm.py", line 77, in getitem",
" return self.subnet_tree[key]",
" File "/usr/local/lib/python2.7/dist-packages/SubnetTree.py", line 157, in getitem",
" return SubnetTree.SubnetTree___getitem(self, cidr)",
"KeyError: '1.0.0.0'",
"",
"----------------------------------------------------------------------",
"Ran 1 test in 0.004s",
"",
"FAILED (errors=1)"

Describe the results you expected:
we expect the route file: fib_info.txt should not be empty!

Additional information you deem important:

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```