M101: OMSDeviceStatusEvent handling triggered from listener in RSN platform driver

@edwardhunter please merge

@bobfrat :

RSN driver listens to the OMSDeviceStatusEvent's (published by the service gateway) for corresponding origin=platform_id, to then notify the corresponding platform agent, which in turn notifies the mission executive for event-driven mission execution purposes.

With

    bin/nosetests -sv  --with-pycc ion/agents/platform/test/test_mission_manager.py:TestPlatformAgentMission.test_simple_mission_ports_on_off

you can see in the logs something like

    ion.agents.platform.mission_manager:73 [mm] Notifying 1 missions about external_event_received: OMSDeviceStatusEvent({'origin': 'LJ01D', 'status': 1, '_id': '2a2427ccbac247ecb77917f1ce62c8a7', 'description': "OMS_EVENT_ID_A's message", 'time_stamps': [], 'valid_values': [], 'persisted_version': 1, 'values': [], 'actor_id': '', 'base_types': ['DeviceStatusEvent', 'DeviceEvent', 'Event'], 'status_details': {'group': '', 'severity': 3, 'listener_url': 'http://localhost:5000/ion-service/oms_event', 'event_id': 'OMS_EVENT_ID_A', 'timestamp': 3609865459.758903, 'platform_id': 'LJ01D', 'message': "OMS_EVENT_ID_A's message", 'first_time_timestamp': 3609865459.758903}, 'ts_created': '1400876659761', 'sub_type': 'OMS_EVENT_ID_A', 'origin_type': 'OMS Platform'}).