[train][goal] Run 'trove-tempest-ipv6-only' job

As part of Train community goal 'Support IPv6-Only Deployments and Testing'[1],
Tempest has defined the base job 'devstack-tempest-ipv6' which will
deploy services on IPv6.

Depends on commit has added the new job 'trove-tempest-ipv6-only'
which is derived from 'devstack-tempest-ipv6'.

Verification structure will be:
- 'devstack-IPv6' deploy the service on IPv6
- 'devstack-tempest-ipv6' run will verify the IPv6-only setting and listen address
- 'trove-tempest-ipv6-only' will run the tests.

Run the ipv6 job on trove gate also.

Depends-On: https://review.opendev.org/#/c/677014/

Story: #2005477
Task: #35937

[1] https://governance.openstack.org/tc/goals/train/ipv6-support-and-testing.html

Change-Id: I2ecfcef4a03ccc574d4fd3b026183342cb1fa242

.zuul.yaml

@@ -37,6 +37,7 @@
         - trove-tox-bandit-baseline:
             voting: false
         - openstack-tox-pylint
+        - trove-tempest-ipv6-only
     gate:
       queue: trove
       jobs:
@@ -45,6 +46,7 @@
         - trove-scenario-mysql-multi
         - trove-tox-fakemodetests
         - openstack-tox-pylint
+        - trove-tempest-ipv6-only
     experimental:
       jobs:
         - trove-grenade