OOIION-626: use DeviceEvent instead of PlatformAlarmEvent

https://jira.oceanobservatories.org/tasks/browse/OOIION-626

Changes done:

For publication of platform events, PlatformAlarmEvent is not used anymore. 
Instead, DeviceEvent is used directly. The attributes in PlatformAlarmEvent previously used,
(external_alarm_type, external_timestamp) are now captured as part of the description,
which now also includes the "group" associated to the event, eg., "power", This group
was previously captured in the event's sub_type. Now the event's sub_type is "platform_event".

NOTE: Using "platform event" termonology instead "platform alarm" because "platform event" is 
the more generic term here (not all platform events are platform alarms).

W.r.t. ion/services/sa/observatory/test/test_oms_launch2.py, the above change is reflected
as follows in the logs:

- publication of platform event:

2012-12-17 15:45:09,561 INFO Dummy-156 ion.agents.platform.platform_agent:661 'LJ01D': publishing external platform event: event_data={'description': 'message: low battery (synthetic event generated from simulator); group: power; external_event_type: 44.78; external_timestamp: 3564776709.55', 'sub_type': 'platform_event'}
2012-12-17 15:45:09,561 TRACE Dummy-156 pyon.event.event:92 Publishing event message to ('ion_test_2c14fa.pyon.events', 'Event.DeviceEvent.platform_event._.TODO_some_platform_id_of_type_UPS')
2012-12-17 15:45:09,561 DEBUG Dummy-156 pyon.net.channel:340 SendChannel.connect: NP (ion_test_2c14fa.pyon.events,Event.DeviceEvent.platform_event._.TODO_some_platform_id_of_type_UPS,B: Event.DeviceEvent.platform_event._.TODO_some_platform_id_of_type_UPS)

- reception of event by subscriber in test:

2012-12-17 15:45:09,572 INFO Dummy-142 ion.services.sa.observatory.test.test_oms_launch2:514 Event subscriber received evt: {'origin': 'TODO_some_platform_id_of_type_UPS', 'description': 'message: low battery (synthetic event generated from simulator); group: power; external_event_type: 44.78; external_timestamp: 3564776709.55', 'type_': 'DeviceEvent', 'base_types': ['Event'], 'ts_created': '1355787909561', 'sub_type': 'platform_event', 'origin_type': ''}.


Before the change, the same elements looked like the following:

- publication of platform event:

2012-12-17 15:21:56,436 INFO Dummy-157 ion.agents.platform.platform_agent:669 'LJ01D': publishing external platform event: event_data={'external_alarm_type': '44.78', 'external_timestamp': 3564775316.4313583, 'ts_created': 3564775316.436277, 'description': 'low battery (synthetic event generated from simulator)', 'sub_type': 'power'}
2012-12-17 15:21:56,436 TRACE Dummy-157 pyon.event.event:92 Publishing event message to ('ion_test_acf8c5.pyon.events', 'Event.DeviceEvent.PlatformEvent.PlatformAlarmEvent.power._.TODO_some_platform_id_of_type_UPS')
2012-12-17 15:21:56,437 DEBUG Dummy-157 pyon.net.channel:340 SendChannel.connect: NP (ion_test_acf8c5.pyon.events,Event.DeviceEvent.PlatformEvent.PlatformAlarmEvent.power._.TODO_some_platform_id_of_type_UPS,B: Event.DeviceEvent.PlatformEvent.PlatformAlarmEvent.power._.TODO_some_platform_id_of_type_UPS)

- reception of event by subscriber in test:

2012-12-17 15:41:22,753 INFO Dummy-142 ion.services.sa.observatory.test.test_oms_launch2:514 Event subscriber received evt: {'origin': 'TODO_some_platform_id_of_type_UPS', 'description': 'low battery (synthetic event generated from simulator)', 'type_': 'PlatformAlarmEvent', 'base_types': ['PlatformEvent', 'DeviceEvent', 'Event'], 'external_alarm_type': '44.78', 'external_timestamp': 3564776482.7359943, 'ts_created': 3564776482.741375, 'sub_type': 'power', 'origin_type': ''}.


 - - - - - - - - 
NOTE regarding NTP:
Maurice: I haven't changed anything related with the use of NTP in the underlying RSN OMS interface. According to our conversation today, this would be a matter of doing conversions so the interface to CI is basically according to whatever is required there, but then translating back a fort the underlying NTP timestamps.  However, I'm doing the pull request with the part already implemented as indicated above as it's about a very specific change in terms of event definitions. If you agree, let's create a separate bug for the NTP conversions. Thanks.