Correct a misuse of RestController in the Event API

When nested RestControllers exist, in order to determine which
arguments are associated with the parent resource, pecan looks at the
get_one() method signature in the parent controller.

TraitsController needs the EventTypesController.get_one method to exist in
order to properly pass the `event_type` identifier.

Change-Id: Ic1704c8f9c810ada4e22e8de75ef2511006e8d9d
Merge "Correct a misuse of RestController in the Event API"