Allow event callback registration for arbitrary resource types

In some cases events should still be able to be registered for
resource types that have not been explicitly defined due to an
extension not being loaded or active. The if-check to raise
a ValueError if the ResourceType has not been explicitly
defined by a @dependancy.provides line has been removed.

Change-Id: I9fb368c5734a4a560a4bdd97d4467e2f76ca0e5c
Merge "Allow event callback registration for arbitrary resource types"