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: Ibad815c50a15aa8df7349262109fcdb2e4a3e341

src/tests/bundles/disco-postgres.yaml

@@ -0,0 +1,13 @@
+series: disco
+services:
+  vault:
+    num_units: 1
+    series: disco
+    charm: ../../../vault
+  postgresql:
+    series: xenial
+    charm: cs:postgresql
+    num_units: 1
+relations:
+- - vault:db
+  - postgresql:db

src/tests/tests.yaml

@@ -8,6 +8,7 @@ gate_bundles:
 - xenial-postgres
 - xenial-mysql
 - bionic-postgres
+- disco-postgres
 smoke_bundles:
 - xenial-mysql
 target_deploy_status:

src/tox.ini

@@ -1,6 +1,10 @@
 [tox]
 envlist = pep8
 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}

tox.ini

@@ -4,7 +4,10 @@
 [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}