[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

devstack/lib/watcher

@@ -193,15 +193,15 @@ function create_watcher_conf {
     iniset_rpc_backend watcher $WATCHER_CONF
 
     iniset $WATCHER_CONF database connection $(database_connection_url watcher)
-    iniset $WATCHER_CONF api host "$WATCHER_SERVICE_HOST"
+    iniset $WATCHER_CONF api host "$(ipv6_unquote $WATCHER_SERVICE_HOST)"
 
     if is_service_enabled tls-proxy; then
-        iniset $WATCHER_CONF api host "$WATCHER_SERVICE_HOST"
+        iniset $WATCHER_CONF api host "$(ipv6_unquote $WATCHER_SERVICE_HOST)"
         iniset $WATCHER_CONF api port "$WATCHER_SERVICE_PORT_INT"
         # iniset $WATCHER_CONF api enable_ssl_api "True"
     else
         if [[ "$WATCHER_USE_WSGI_MODE" == "mod_wsgi" ]]; then
-            iniset $WATCHER_CONF api host "$WATCHER_SERVICE_HOST"
+            iniset $WATCHER_CONF api host "$(ipv6_unquote $WATCHER_SERVICE_HOST)"
             iniset $WATCHER_CONF api port "$WATCHER_SERVICE_PORT"
         fi
     fi