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 small additions:
- Passing OS env vars instead of AMULET
- removing sitepackages in tox.ini to avoid test env pollution
- skip_missing_interpreters in tox.ini set to False to avoid false
positives by skipping missing interpreters.
- adding bionic-stein bundle to gate_bundles instead of dev

LP Related-Bug: #1823718
Change-Id: Ia7093a5654a763670be05635f7f6fce292eb9c96