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.

Change-Id: I8d5af206e2af881013b1f78f7cc913036e6b23c2

tox.ini

@@ -2,6 +2,10 @@
 skipsdist = True
 envlist = pep8
 toxworkdir = /tmp/tox
+# 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]
 basepython = python3
@@ -17,6 +21,7 @@ deps =
 whitelist_externals = /bin/true /bin/echo /bin/mkdir
 
 [testenv:build]
+basepython = python3
 commands =
   /bin/echo 'WARNING: *build* target is for testing only.'
   /bin/mkdir -p {envdir}/tmp