Fix NoSuchOptError in lbaas agent test

Option 'periodic_interval' doesn't exist when using testtools
to test lbaas agent, fix this by adding a mock object.

Change-Id: I5ef02d6fa154ea7a18ca77d02a2704b7e3990c3b
Closes-Bug: #1264260

neutron/tests/unit/services/loadbalancer/drivers/haproxy/test_agent.py

@@ -35,6 +35,7 @@ class TestLbaasService(base.BaseTestCase):
         ) as mock_start:
 
             mgr = mock.Mock()
+            cfg.CONF.periodic_interval = mock.Mock(return_value=10)
             agent_service = agent.LbaasAgentService('host', 'topic', mgr)
             agent_service.start()