Add stable/wallaby jobs on master gate

We have stable/wallaby released so we should add
their job on master gate to keep branchless tempest
plugins compatible to stable branch.

This also removes the stable/stein job as that is in EM
state now.

Ref: Tempest plugins guide for stable branch testing:
- https://docs.openstack.org/tempest/latest/stable_branch_testing_policy.html

Change-Id: I9c36e08e8d8e27a97605466953fa79b799fae341
1 file changed