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:
- 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.

LP Related-Bug: #1823718
Change-Id: I37308917d3bb31a7c53fce6a10addf95a85bb2b5

tox.ini

@@ -4,6 +4,10 @@
 [tox]
 envlist = pep8,py37
 skipsdist = True
+# NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
+sitepackages = False
+# NOTE(beisner): Avoid false positives by not skipping missing interpreters.
+skip_missing_interpreters = False
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}