Switch testing to Xena testing runtime

Upating the tetsing template to Xena testing runtime:
https://governance.openstack.org/tc/reference/runtimes/xena.html

Change-Id: I18fa6b0f12114df57376c47f6e185796505572dc

.zuul.yaml

@@ -3,4 +3,4 @@
       - check-requirements
       - publish-openstack-docs-pti
       - openstack-python35-jobs
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-xena-jobs