Add stable branches testing on keystone-tempest-plugin master gate

This patch add the jobs for stable/stein, stable/train and
stable/ussuri in keystone tempest plugin. Supported stable branch
use keystone-tempest-plugin master version to test them. Adding stable
job on master ensures that keystone-tempest-plugin master version is
compatible with stable branches testing.

Change-Id: I72bf38247f693a2efcdad2e64a8948023350ff53
1 file changed