Update tox to correctly use OS_TEST_PATH

Currently, to run unit tests, Patrole relies on a whitelist file
to separate unit tests from API tests so that tox -e py27/35 works.
However, this approach is hackish and has been abandoned in
favor of the following standard approach:

  - Use OS_TEST_PATH to .testr.conf
  - Override OS_TEST_PATH for unit test jobs in tox.ini
  - Remove the whitelist file as it's no longer needed

Change-Id: Iedbd01b539d24dc57e3c8bc1bb0dbb3ea9127ef4
3 files changed