Make tox use develop instead of sdist

tox 1.6 was released, which means that we can now take advantage of the
feature we added to it - which is using setup.py develop to install the
code into the virtualenv. The logic was taken from run_tests.sh - so the
performance issues around using tox vs. using install_venv should now be
gone.

This patch requires tox >=1.6.

Change-Id: I505fb530e8047f478e98ce2fd6c01776b147d87f

tox.ini

@@ -1,7 +1,12 @@
 [tox]
+minversion = 1.6
 envlist = py26,py27,py33,pypy,pep8
+skipsdist = True
 
 [testenv]
+usedevelop = True
+# Customize pip command, add -U to force updates.
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          NOSE_WITH_OPENSTACK=1
          NOSE_OPENSTACK_COLOR=1