Replace run_tests.sh helper script with tox.

 * Added installation of tox - the preferred way to repeatedly
   run tempest.
 * Removed unnecessary run_tests.sh script - replaced by tox.
 * Added setup_venv parameter to enable automatic creation of a
   venv installed with tempest dependencies for debugging purposes.

Change-Id: I729d041a606810ba2e60543855a44f0e10870c0b