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
diff --git a/.testr.conf b/.testr.conf
index 6d83b3c..87d049d 100644
--- a/.testr.conf
+++ b/.testr.conf
@@ -2,6 +2,6 @@
 test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
              OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
              OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
-             ${PYTHON:-python} -m subunit.run discover -t ./ . $LISTOPT $IDOPTION
+             ${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./patrole_tempest_plugin/tests/unit} $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list