Add Tempest 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 Tempest testing for stable/rocky
by adding new jobs running on stable/rocky version of
openstack.
Depends-On: https://review.openstack.org/#/c/596276/

Change-Id: I88d4b121bf461495b357916f4b7827b46a2fbc46
1 file changed