[train][goal] Define new 'watcher-tempest-functional-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.

This commit adds the new job 'watcher-tempest-functional-ipv6-only'
run on gate 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
- 'watcher-tempest-functional-ipv6-only' will run the tests.

Story: #2005477
Task: #35939

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

Change-Id: I42b7e5ff5fd64a21bdb8a32f319759a18c173601

.zuul.yaml

@@ -21,10 +21,12 @@
         - watcher-tempest-host_maintenance
         - watcher-tempest-storage_balance
         - watcher-tls-test
+        - watcher-tempest-functional-ipv6-only
     gate:
       queue: watcher
       jobs:
         - watcher-tempest-functional
+        - watcher-tempest-functional-ipv6-only
 
 - job:
     name: watcher-tempest-dummy_optim
@@ -160,7 +162,7 @@
     name: watcher-tempest-functional
     parent: devstack-tempest
     timeout: 7200
-    required-projects:
+    required-projects: &base_required_projects
       - openstack/ceilometer
       - openstack/devstack-gate
       - openstack/python-openstackclient
@@ -168,7 +170,7 @@
       - openstack/watcher
       - openstack/watcher-tempest-plugin
       - openstack/tempest
-    vars:
+    vars: &base_vars
       devstack_plugins:
         watcher: https://opendev.org/openstack/watcher
       devstack_services:
@@ -191,6 +193,14 @@
       zuul_copy_output:
         /etc/hosts: logs
 
+- job:
+    name: watcher-tempest-functional-ipv6-only
+    parent: devstack-tempest-ipv6
+    description: |
+      Watcher devstack tempest tests job for IPv6-only deployment
+    required-projects: *base_required_projects
+    vars: *base_vars
+
 - job:
     name: watcher-grenade
     parent: legacy-dsvm-base