Create rbac utils fixture and refactor tests

This commit creates a test fixture for rbac_utils.RbacUtils
and refactors test_rbac_utils tests accordingly. This
allows other tests to use the fixture as needed, improving
code maintenance and readability, as well as the ease
with which tests can be written.

Specifically:
  - adds fixtures file for overriding conf settings and
    for rbac_utils.RbacUtils
  - refactors rbac_utils tests to use the appropriate
    fixture

Change-Id: I2f49137ff71089ecf9764ee1f7887b64185249dd
3 files changed