Have tox install via setup.py develop

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.

run_tests.sh is still needed to handle the ui switching between
parallel, single and debug modes to testr.

Change-Id: Ida5e440d1bdb9f8e9031277ea53a02d2ef171438

tox.ini

@@ -1,18 +1,21 @@
 [tox]
+minversion = 1.6
 envlist = py26,py27,py33,pep8
+skipsdist = True
 
 [testenv]
 sitepackages = True
+usedevelop = True
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C
-deps = -U
-       -r{toxinidir}/requirements.txt
+deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   python tools/patch_tox_venv.py
-  python setup.py testr --slowest --testr-args='{posargs}'
+  python setup.py test --slowest --testr-args='{posargs}'
 
 [tox:jenkins]
 sitepackages = True