platform agent/driver/ci-oms adjustments and more clean-up

- more clean-up related with platform ports and OOIION-1495, including fixes to potential references to NoneType as a dict
- get port status in platform driver via request to RSN OMS (not from config)
- platform attribute definitions retrieved from call to platform driver (not from config)
- raise IonException-derived exceptions in PlatformAgent's on_init and on_start upon any problems with configuration. This fixes misalignment with expected behavior.

@edwardhunter please review/merge