Avoid pollution and false positives with tox.ini

- Added 'Alternatives' section in lxd-to-nova-lxd-rename.rst too avoid
pep8 error, and fixed other pep8 mistakes
- changing testenv to python3
- 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: Id746fff4bfe7b0dd3ccb2803793f78a194d81355

specs/stein/approved/lxd-to-nova-lxd-rename.rst

@@ -52,6 +52,10 @@ The charm name will be switched with the ``juju upgrade-charm lxd
 --switch=nova-lxd`` method of changing a charm and this will be tested
 during upgrades.
 
+Alternatives
+------------
+
+No alternatives suggested.
 
 Implementation
 ==============

specs/train/backlog/cinder-backup-swift.rst

@@ -68,7 +68,8 @@ a backend for volume backups.
 
   - backup_swift_ca_cert_file
 
-Implement a new interface cinder-backup in reactive framework for sending cinder-backup external backend configuration.
+Implement a new interface cinder-backup in reactive framework for sending
+cinder-backup external backend configuration.
 
 Alternatives
 ------------

tox.ini

@@ -2,11 +2,15 @@
 # Hold back to 1.4, since that's what's in Fedora 20 repos
 # and we don't need anything newer for charm-specs tests
 minversion = 1.4
-envlist = docs,py27,pep8
+envlist = docs,py3,pep8
 skipsdist = 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]
-basepython = python2.7
+basepython = python3
 usedevelop = True
 setenv =
   VIRTUAL_ENV={envdir}