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

zuul.d/project.yaml

@@ -31,6 +31,8 @@
         - glance-multistore-cinder-import:
             voting: false
             irrelevant-files: *tempest-irrelevant-files
+        - tempest-full-xena:
+            irrelevant-files: *tempest-irrelevant-files
         - tempest-full-wallaby-py3:
             irrelevant-files: *tempest-irrelevant-files
         - tempest-full-victoria-py3:
@@ -150,6 +152,7 @@
             irrelevant-files: *tempest-irrelevant-files
     periodic-stable:
       jobs:
+        - tempest-full-xena
         - tempest-full-wallaby-py3
         - tempest-full-victoria-py3
         - tempest-full-ussuri-py3

zuul.d/stable-jobs.yaml

@@ -1,4 +1,9 @@
 # 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