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
diff --git a/test-requirements.txt b/test-requirements.txt
index 177d0fd..34b2238 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -6,6 +6,7 @@
 sphinx>=1.6.2 # BSD
 openstackdocstheme>=1.11.0 # Apache-2.0
 reno!=2.3.1,>=1.8.0 # Apache-2.0
+fixtures>=3.0.0  # Apache-2.0/BSD
 mock>=2.0 # BSD
 coverage!=4.4,>=4.0 # Apache-2.0
 nose # LGPL