Avoid pollution and false positives with tox.ini

- 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.

- add missing zuul.yaml to configure check/gate tests.

Change-Id: I60a7b5163c311ba929e8b44d2ff022bb75423b3c

.gitreview

@@ -0,0 +1,4 @@
+[gerrit]
+host=review.opendev.org
+port=29418
+project=openstack/charm-interface-cinder-backend

.zuul.yaml

@@ -0,0 +1,4 @@
+- project:
+    templates:
+      - python35-charm-jobs
+      - openstack-python3-train-jobs

tox.ini

@@ -1,7 +1,10 @@
 [tox]
 skipsdist = True
 envlist = pep8,py3
-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}
@@ -15,12 +18,18 @@ basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
+[testenv:py35]
+basepython = python3.5
+deps = -r{toxinidir}/test-requirements.txt
+commands = ostestr {posargs}
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} .
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]