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>
(cherry picked from commit e941f9c5487cdfc6f5bfe87ce382e7ff77c588c1)
Signed-off-by: Robert Varga <rovarga@cisco.com>