Add min env vars doc validation to pep8 gate

Change-Id: I7711e9ee597184e34c88c0deeaf916fb1547b704
Implements: blueprint add-min-env-vars-gate

tox.ini

@@ -13,6 +13,7 @@ commands =
     {toxinidir}/tools/validate-all-yaml.sh
     {toxinidir}/tools/validate-all-maintainer.sh
     {toxinidir}/tools/validate-links.sh
+    {toxinidir}/tools/validate-min-env-vars.sh
 
 [testenv:bashate]
 deps = bashate