redis driver raise exception when node removed

set enable_port_status_notifier=True for neutron-server, then remove a redis node
2016-11-17 17:37:19.847 115759 WARNING dragonflow.db.pubsub_drivers.redis_db_pubsub_driver [req-bdb4503b-73e5-4362-98ae-b56b66de9dd3 - -] subscriber listening task lost connection Error 111 connecting to 10.248.92.227:4002. ECONNREFUSED.
2016-11-17 17:37:19.848 115759 INFO dragonflow.db.drivers.redis_mgt [req-bdb4503b-73e5-4362-98ae-b56b66de9dd3 - -] remove node 10.248.92.227:4002 from redis master list
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/eventlet/greenpool.py", line 82, in _spawn_n_impl
    func(*args, **kwargs)
  File "/opt/stack/dragonflow/dragonflow/db/pubsub_drivers/redis_db_pubsub_driver.py", line 239, in run
    self.register_hamsg_for_db()
  File "/opt/stack/dragonflow/dragonflow/db/pubsub_drivers/redis_db_pubsub_driver.py", line 179, in register_hamsg_for_db
    self.redis_mgt.register_ha_topic()
  File "/opt/stack/dragonflow/dragonflow/db/drivers/redis_mgt.py", line 334, in register_ha_topic
    if self.subscriber is not None:
AttributeError: 'RedisMgt' object has no attribute 'subscriber'
2016-11-17 17:37:22.636 115759 INFO dragonflow.db.pubsub_drivers.redis_db_pubsub_driver [-] publish connection old masterlist [{'ip_port': '10.248.92.227:4001', 'slot': [[0, 5460]]}, {'ip_port': '10.248.92.227:4003', 'slot': [[10923, 16383]]}]
2016-11-17 17:37:22.637 115761 WARNING dragonflow.db.drivers.redis_mgt [-] redis cluster nodes less than local, maybe there is a partition in db cluster, nodes:{'10.248.92.227:4001': {'status': 'connected', 'last_pong_rcvd': '0', 'epoch': '1', 'node_id': '023d96311c63155a039cc405a3a6d039d17ac7fc', 'role': 'master', 'last_ping_sent': '0', 'slots': [['0', '5460']], 'master_id': '-'}, '10.248.92.227:4003': {'status': 'connected', 'last_pong_rcvd': '1479375437618', 'epoch': '3', 'node_id': 'c05794146cf2beffe6ca3f51ed673829e4711ec1', 'role': 'master', 'last_ping_sent': '0', 'slots': [['10923', '16383']], 'master_id': '-'}, '10.248.92.227:4002': {'status': 'disconnected', 'last_pong_rcvd': '1479375439649', 'epoch': '2', 'node_id': '1db300860fe8bffa9019e1a5185fc29821e6c44f', 'role': 'master', 'last_ping_sent': '1479375439926', 'slots': [['5461', '10922']], 'master_id': '-'}, '10.248.92.227:4004': {'status': 'connected', 'last_pong_rcvd': '1479375441633', 'epoch': '4', 'node_id': '4f982c8410505830ea0fe5b99bddeeadd111a17d', 'role': 'slave', 'last_ping_sent': '0', 'slots': [], 'master_id': '023d96311c63155a039cc405a3a6d039d17ac7fc'}, '10.248.92.227:4006': {'status': 'connected', 'last_pong_rcvd': '1479375441131', 'epoch': '6', 'node_id': '704857fbe4665e099d5fc7e8d3b48e8f6e64309c', 'role': 'slave', 'last_ping_sent': '0', 'slots': [], 'master_id': 'c05794146cf2beffe6ca3f51ed673829e4711ec1'}}, local nodes:{'10.248.92.227:4001': {'status': 'connected', 'last_pong_rcvd': '0', 'epoch': '1', 'node_id': '023d96311c63155a039cc405a3a6d039d17ac7fc', 'role': 'master', 'last_ping_sent': '0', 'slots': [['0', '5460']], 'master_id': '-'}, '10.248.92.227:4003': {'status': 'connected', 'last_pong_rcvd': '1479375234189', 'epoch': '3', 'node_id': 'c05794146cf2beffe6ca3f51ed673829e4711ec1', 'role': 'master', 'last_ping_sent': '0', 'slots': [['10923', '16383']], 'master_id': '-'}, '10.248.92.227:4002': {'status': 'connected', 'last_pong_rcvd': '1479375232183', 'epoch': '2', 'node_id': '1db300860fe8bffa9019e1a5185fc29821e6c44f', 'role': 'master', 'last_ping_sent': '0', 'slots': [['5461', '10922']], 'master_id': '-'}, '10.248.92.227:4005': {'status': 'connected', 'last_pong_rcvd': '1479375235191', 'epoch': '5', 'node_id': '2e4d50cc44daf56324fa21595cdfe18c2bd1e4c5', 'role': 'slave', 'last_ping_sent': '0', 'slots': [], 'master_id': '1db300860fe8bffa9019e1a5185fc29821e6c44f'}, '10.248.92.227:4004': {'status': 'connected', 'last_pong_rcvd': '1479375231180', 'epoch': '4', 'node_id': '4f982c8410505830ea0fe5b99bddeeadd111a17d', 'role': 'slave', 'last_ping_sent': '0', 'slots': [], 'master_id': '023d96311c63155a039cc405a3a6d039d17ac7fc'}, '10.248.92.227:4006': {'status': 'connected', 'last_pong_rcvd': '1479375233186', 'epoch': '6', 'node_id': '704857fbe4665e099d5fc7e8d3b48e8f6e64309c', 'role': 'slave', 'last_ping_sent': '0', 'slots': [], 'master_id': 'c05794146cf2beffe6ca3f51ed673829e4711ec1'}}
2016-11-17 17:37:22.637 115759 INFO dragonflow.db.pubsub_drivers.redis_db_pubsub_driver [-] publish connection new masterlist [{'ip_port': '10.248.92.227:4001', 'slot': [[0, 5460]]}, {'ip_port': '10.248.92.227:4003', 'slot': [[10923, 16383]]}]