Add rbac_utils is_admin helper method

This patch adds a helper method to rbac_utils.RbacUtils called
``is_admin`` which returns True if the current RBAC test role
is equal to the admin role defined in tempest.conf (i.e.
CONF.rbac.rbac_test_role == CONF.identity.admin_role). This
makes it easier to check this equivalence in a test by simply
running:

    if self.rbac_utils.is_admin:
        do something

Change-Id: I0efb005f90ac77449453f6b68a010ec64e7a03f7
diff --git a/patrole_tempest_plugin/rbac_utils.py b/patrole_tempest_plugin/rbac_utils.py
index fe2d99f..3bb2cbd 100644
--- a/patrole_tempest_plugin/rbac_utils.py
+++ b/patrole_tempest_plugin/rbac_utils.py
@@ -162,3 +162,11 @@
 
         self.admin_role_id = admin_role_id
         self.rbac_role_id = rbac_role_id
+
+    @property
+    def is_admin(self):
+        """Verifies whether the current test role equals the admin role.
+
+        :returns: True if ``rbac_test_role`` is the admin role.
+        """
+        return CONF.rbac.rbac_test_role == CONF.identity.admin_role