Fix Neutron Gateway deferred restart test

Do not run the deferred hooks action for neutron gateway as it is
not needed and will end in an action error.

Tested locally here: https://paste.ubuntu.com/p/cSmgxmrTr4/

This closes issue #553

zaza/openstack/charm_tests/neutron/tests.py

@@ -1045,3 +1045,7 @@ class NeutronGatewayDeferredRestartTest(test_utils.BaseDeferredRestartTest):
         self.run_package_change_test(
             'openvswitch-switch',
             'openvswitch-switch')
+
+    def check_clear_hooks(self):
+        """Gateway does not defer hooks so noop."""
+        return