[train][goal] Define new tricircle-functional-ipv6-only job

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

This commit adds the new job 'tricircle-functional-ipv6-only'
run on gate which is derived from 'legacy-dsvm-base-ipv6-only'.

Verification structure will be:
- 'legacy-dsvm-base-ipv6-only' deploy the service on IPv6 and
  verify the IPv6-only setting and listen address
- 'tricircle-functional-ipv6-only' will run the tests.

Story: #2005477
Task: #35935

Depends-On: https://review.opendev.org/#/c/680233/
[1] https://governance.openstack.org/tc/goals/train/ipv6-support-and-testing.html

Change-Id: I5f430b4ae7690ae80d8a640b3706337036778029

.zuul.yaml

@@ -25,6 +25,18 @@
       devstack_localrc:
         USE_PYTHON3: true
 
+- job:
+    name: tricircle-functional-ipv6-only
+    parent: legacy-dsvm-base-ipv6-only
+    run: playbooks/tricircle-dsvm-functional/run.yaml
+    post-run: playbooks/tricircle-dsvm-functional/post.yaml
+    timeout: 7800
+    required-projects:
+      - openstack/devstack-gate
+      - openstack/tricircle
+      - openstack/neutron
+      - openstack/networking-sfc
+
 - job:
     name: tricircle-multiregion
     parent: legacy-dsvm-base-multinode
@@ -77,6 +89,7 @@
               - openstack/networking-sfc
         - tricircle-functional
         - tricircle-functional-python3
+        - tricircle-functional-ipv6-only
         - tricircle-multiregion
     gate:
       jobs:
@@ -94,4 +107,5 @@
               - openstack/neutron
               - openstack/networking-sfc
         - tricircle-functional
+        - tricircle-functional-ipv6-only
         - tricircle-multiregion