ONOS aaa-app Device Disable/Delete Event Handling

Behaviour:
ONU Disable:
Current and Expected: PORT_DOWN: nothing to be done
ONU Delete:
Current and Expected: PORT_REMOVED: purge everything (AAA state, flows etc.) for that session
OLT Disable:
Current and Expected: PORT_DOWN: nothing to be done
OLT Delete:
Current: DEVICE_AVAILABILITY_CHANGED: purging all session information but not the authentications map.
Expected: PORT_REMOVED: To be taken care by VOL-2573
Also, for event like DEVICE_REMOVED: can only be called by ONOS cli, nothing in vOLTHA causes this: purge all the stores/maps etc. (and to be used only after OLT Delete)