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: I6596739f10292d3953834d56c24de380d405b90b

tox.ini

@@ -4,6 +4,10 @@
 [tox]
 envlist = pep8,py27,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}
@@ -14,7 +18,7 @@ install_command =
   pip install {opts} {packages}
 commands = stestr run {posargs}
 whitelist_externals = juju
-passenv = HOME TERM AMULET_* CS_API_*
+passenv = HOME TERM AMULET_* CS_API_* OS_*
 
 [testenv:py27]
 basepython = python2.7