BUG-868: Require ObjectRegistration in CompositeObjectRegistration

With Registration becoming an abstract concept, force users to use
ObjectRegistration.

Change-Id: I7baa722d2adf3e399a0c477b303983b92fbd5fcf
Signed-off-by: Robert Varga <rovarga@cisco.com>
BUG-868: Move getInstance() from Registration into ObjectRegistration

This finishes up the conversion of Registration into an
object-independent concept.

Change-Id: Id6a27004de23ca6faff31303304e1171a8552d9f
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge changes Id6a27004,I7baa722d

* changes:
  BUG-868: Move getInstance() from Registration into ObjectRegistration
  BUG-868: Require ObjectRegistration in CompositeObjectRegistration