Switch to Ussuri jobs

Change-Id: I01c8b66665434397115f59012e325e710567085b

zuul.d/layout.yaml

@@ -3,5 +3,5 @@
       - check-requirements
       - openstack-cover-jobs
       - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs
       - tripleo-multinode-container-minimal