Add disco-stein bundle to gate tests

This project does not have func tests, however these changes are in line
with a batch of changes over all charms, so I kept the same title for
reference.
Changes:
- 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.

Change-Id: I5241afe9b0bdcfd8b7cd61d1395259d0bb96cae2

tox.ini

@@ -1,7 +1,10 @@
 [tox]
 skipsdist = True
 envlist = pep8,py37
-skip_missing_interpreters = 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}