Run 'tempest-ipv6-only' job in gate

As part of Train community goal 'Support IPv6-Only Deployments and Testing'[1],
Tempest has defined the new job 'tempest-ipv6-only'(adding
in Depends-On patch) which will deploy services on IPv6 and run smoke
tests and IPv6 related tests present in Tempest.

This job will be part of Nova, Neutron, Cinder, Keystone, Glance, Swift
gate.

Verification structure will be:
- 'devstack-IPv6' deploy the service on IPv6
- 'devstack-tempest-ipv6' run will verify the IPv6-only setting and listen address
- 'tempest-ipv6-only' will run the smoke + IPv6 related test case.

This commit adds the new job 'tempest-ipv6-only' run on gate.

Story: #2005477
Task: #35891

Depends-On: https://review.opendev.org/#/c/671231/

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

Change-Id: Ibe2c01713df2bec366c49d6e41b65b818b5e98e8

.zuul.yaml

@@ -261,6 +261,8 @@
             irrelevant-files: *tempest-irrelevant-files
         - grenade-py3:
             irrelevant-files: *tempest-irrelevant-files
+        - tempest-ipv6-only:
+            irrelevant-files: *tempest-irrelevant-files
 
     gate:
       jobs:
@@ -274,6 +276,8 @@
             irrelevant-files: *tempest-irrelevant-files
         - grenade-py3:
             irrelevant-files: *tempest-irrelevant-files
+        - tempest-ipv6-only:
+            irrelevant-files: *tempest-irrelevant-files
     experimental:
       jobs:
         - glance-tox-py27-glance_store-tips