[IPv6 only] support IPv6 only test case for KVM testbed. about sonic-mgmt

[sdszhang]

After enabling IPv6 for docker daemon, ip/test_mgmt_ipv6_only.py are passed:

1. change the docker daemon config as following:
   $cat /etc/docker/daemon.json
   {
   "ipv6": true,
   "fixed-cidr-v6": "2001:db8:1::/64",
   "experimental": true,
   "ip6tables": true,
   }
2. restart docker container
   $sudo systemctl restart docker
3. delete the current sonic-mgmt docker using command docker rm -f <sonic-mgmt_container_name>
4. setup the sonic-mgmt container, then run ip/test_mgmt_ipv6_only.py test case.

ip/test_mgmt_ipv6_only.py::test_bgp_facts_ipv6_only[vlab-01-None] PASSED [ 14%]
ip/test_mgmt_ipv6_only.py::test_show_features_ipv6_only[vlab-01] PASSED [ 28%]
ip/test_mgmt_ipv6_only.py::test_image_download_ipv6_only[vlab-01] SKIPPED (Cannot get image url) [ 42%]
ip/test_mgmt_ipv6_only.py::test_syslog_ipv6_only[fd82:b34f:cc99::100-None] PASSED [ 57%]
ip/test_mgmt_ipv6_only.py::test_syslog_ipv6_only[fd82:b34f:cc99::100-fd82:b34f:cc99::200] PASSED [ 71%]
ip/test_mgmt_ipv6_only.py::test_ro_user_ipv6_only[vlab-01] PASSED [ 85%]
ip/test_mgmt_ipv6_only.py::test_rw_user_ipv6_only[vlab-01] PASSED [100%]

[sdszhang]

documents updated, and ipv6 PR pipeline is enabled. good to close.