[TrivialFix] Use _override_role in rbac_rule_validation

This PS changes switch_role usage to _override_role usage in
rbac_rule_validation. This is a temporary change while switch_role
goes through a deprecation phase. This code change should have been
made earlier in:

    I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I7154b7bf9440aaef69c8dae9b6643f956564369f
diff --git a/patrole_tempest_plugin/rbac_rule_validation.py b/patrole_tempest_plugin/rbac_rule_validation.py
index 82bc1a0..fc8b145 100644
--- a/patrole_tempest_plugin/rbac_rule_validation.py
+++ b/patrole_tempest_plugin/rbac_rule_validation.py
@@ -175,10 +175,10 @@
                         "OverPermission: Role %s was allowed to perform %s" %
                         (role, rule))
             finally:
-                # TODO(felipemonteiro): Remove the `switch_role` call below
-                # once all the tests have migrated over to `override_role`.
-                test_obj.rbac_utils.switch_role(test_obj,
-                                                toggle_rbac_role=False)
+                # TODO(felipemonteiro): Remove the call below once all the
+                # tests have migrated over to `override_role` public method.
+                test_obj.rbac_utils._override_role(test_obj,
+                                                   toggle_rbac_role=False)
                 if CONF.patrole_log.enable_reporting:
                     RBACLOG.info(
                         "[Service]: %s, [Test]: %s, [Rule]: %s, "