Add stable ussuri job on master gate

stable/ussuri branch is setup now
- https://review.opendev.org/#/q/topic:qa-ussuri-release+status:merged+projects:openstack/devstack

and Tempest and plugins master version is used to test the stable/ussuri.

Adding a stable job for ussuri on master so that any new tests
or changes does not break the stable ussuri testing.

Tempest already added the stable/ussuri job on master gate so adding it
for plugins is safe now.
- https://review.opendev.org/#/c/723620/

Change-Id: If8afdbd9b933bf1b07bef1dec54f54a46917d9fd
1 file changed