Use Python3 yoga unit tests

Because the Xena release is not being created for TripleO repos, test
runtime is not updated by automation tools. This change updates
the job template to use the tested runtime for Yoga.

Change-Id: Ia0e65d80302ba55a138c7621e7c6b73de6647060

zuul.d/layout.yaml

@@ -3,7 +3,7 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-yoga-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
       - tripleo-multinode-container-minimal-pipeline