Add gate job for stable/rocky

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

This commit adds the stable/rocky job running for
d-g gate.
Depends-On: https://review.openstack.org/#/c/596279/

Change-Id: I7ab1b7042808cce0d89cb2bcfc87c718543718b4

.zuul.yaml

@@ -19,5 +19,11 @@
         - tempest-full-queens-py3:
             branches: ^(?!driverfixes/).*$
             voting: false
+        - tempest-full-rocky:
+            branches: ^(?!driverfixes/).*$
+            voting: false
+        - tempest-full-rocky-py3:
+            branches: ^(?!driverfixes/).*$
+            voting: false
         - neutron-tempest-plugin-api:
             voting: false