Add Tempest gate job for stable/xena

Now we have stable/xena branch ready for devstack
and so does for all service projects.

This commit adds the Tempest testing for stable/xena
by adding new jobs running on stable/xena version of
openstack.

Change-Id: If6f188aebe48f17d8a343c958cdeb5507421a40b
diff --git a/zuul.d/stable-jobs.yaml b/zuul.d/stable-jobs.yaml
index 852bafb..e682457 100644
--- a/zuul.d/stable-jobs.yaml
+++ b/zuul.d/stable-jobs.yaml
@@ -1,5 +1,10 @@
 # NOTE(gmann): This file includes all stable release jobs definition.
 - job:
+    name: tempest-full-xena
+    parent: tempest-full-py3
+    override-checkout: stable/xena
+
+- job:
     name: tempest-full-wallaby-py3
     parent: tempest-full-py3
     override-checkout: stable/wallaby