Add disco-stein bundle to gate tests.

The bundle disco-stein was in the dev_bundles because
a bug with the Python 3.7 syntax prevented the success of
the deployment of HA clusters. This bug is fixed, we can
reintegrate disco-stein in the gate_bundles.

Other minor adds:
- Changing naming convention for vip: 'Amulet' is deprecated.
- Passing OS env vars

LP Related-Bug: #1823719
Change-Id: I787248ad2983858c6db2e5a838270a128dae717a