Update tox config

An updated tox config is needed to deal with accidental installs of
pre-release software we don't want. Additionally, this update should
make running tox tests quicker due to skipping the sdist.

Closes-Bug: 1233357
Change-Id: I814850d7e108a3502297bbbdda506e260d2a86cb

tox.ini

@@ -1,7 +1,11 @@
 [tox]
+minversion = 1.6
+skipsdist = True
 envlist = py26,py27,py33,pep8
 
 [testenv]
+usedevelop = True
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          NOSE_WITH_OPENSTACK=1
          NOSE_OPENSTACK_COLOR=1