Description

Current KVM testbed doesn't support IPv6 only testcases.

Steps to reproduce the issue:
In sonit-mgmt container,
./run_tests.sh -n vms-kvm-t0 -d vlab-01 -c ip/test_mgmt_ipv6_only.py -f vtestbed.yaml -i ../ansible/veos_vtb

Describe the results you received:
===================================================================================== short test summary info ======================================================================================
ERROR ip/test_mgmt_ipv6_only.py::test_bgp_facts_ipv6_only[vlab-01-None] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_show_features_ipv6_only[vlab-01] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_image_download_ipv6_only[vlab-01] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_syslog_ipv6_only[fd82:b34f:cc99::100-None] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_syslog_ipv6_only[fd82:b34f:cc99::100-fd82:b34f:cc99::200] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_ro_user_ipv6_only[vlab-01] - Failed: vlab-01 doesn't have available IPv6 Management IP address
ERROR ip/test_mgmt_ipv6_only.py::test_rw_user_ipv6_only[vlab-01] - Failed: vlab-01 doesn't have available IPv6 Management IP address

Describe the results you expected:
All test passes.

Additional information you deem important:

**Output of `show version`:**

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

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

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