Add Tempest gate job for stable/stein

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

This commit adds the Tempest testing for stable/stein
by adding new jobs running on stable/stein version of
openstack.
Depends-On: https://review.openstack.org/#/c/647867/

Change-Id: Ia22988821b5c786a0b25a980cb4ca329dac54bac
1 file changed