Change rbac_utils.RbacUtils is_admin to function

This is a trivial change that changes is_admin to a function
(instead of a property) because properties can only be used
in the instance context whereas non-class functions can be used
in both instance and class contexts.

This allows ``test_tokens_negative_rbac`` to use is_admin()
now since it is used in a class context.

Change-Id: I9244f10cd536050986185651421a446fef8e2ade
diff --git a/patrole_tempest_plugin/rbac_rule_validation.py b/patrole_tempest_plugin/rbac_rule_validation.py
index 51b9d92..da27a20 100644
--- a/patrole_tempest_plugin/rbac_rule_validation.py
+++ b/patrole_tempest_plugin/rbac_rule_validation.py
@@ -25,6 +25,7 @@
 
 from patrole_tempest_plugin import rbac_exceptions
 from patrole_tempest_plugin import rbac_policy_parser
+from patrole_tempest_plugin import rbac_utils
 from patrole_tempest_plugin import requirements_authority
 
 CONF = config.CONF
@@ -85,7 +86,7 @@
                 LOG.info("As admin_only is True, only admin role should be "
                          "allowed to perform the API. Skipping oslo.policy "
                          "check for policy action {0}.".format(rule))
-                allowed = test_obj.rbac_utils.is_admin
+                allowed = rbac_utils.is_admin()
             else:
                 allowed = _is_authorized(test_obj, service, rule,
                                          extra_target_data)