Add 'tempest-ipv6-only' job on devstack gate

We recently added the 'tempest-ipv6-only' job on
tempest side which will use the devstack base job
'devstack-ipv6' job and add more verification and testing
for IPv6 deployment.

Let's add that job on devstack gate also to avoid any
break due to devstack changes.

Change-Id: Ib2c85ec262b027351872e2b5a39b06a4ba1b880a
Story: #2005477
Task: #35923
diff --git a/.zuul.yaml b/.zuul.yaml
index 875aed3..9853798 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -657,6 +657,10 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
+        - tempest-ipv6-only:
+            irrelevant-files:
+              - ^.*\.rst$
+              - ^doc/.*$
     gate:
       jobs:
         - devstack
@@ -682,6 +686,10 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
+        - tempest-ipv6-only:
+            irrelevant-files:
+              - ^.*\.rst$
+              - ^doc/.*$
     # Please add a note on each job and conditions for the job not
     # being experimental any more, so we can keep this list somewhat
     # pruned.