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: I4d7814a557bb17afb932ddcff6c75f2a8f61f51e

tox.ini

@@ -1,6 +1,10 @@
 [tox]
 skipsdist = True
 envlist = pep8
+# 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 = python2.7
@@ -16,6 +20,7 @@ deps =
 whitelist_externals = /bin/true /bin/echo
 
 [testenv:build]
+basepython = python3
 commands =
   /bin/echo 'WARNING: *build* target is for testing only.'
   charm-build --log-level DEBUG -o {toxinidir}/build .