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: I2e1f234ff5dcef017cd6e658d303ae3261f2e3b9

tox.ini

@@ -1,7 +1,10 @@
 [tox]
 envlist = pep8,py27,py34,py35
 skipsdist = True
-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}