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
Merge "Have tox install via setup.py develop"