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>
(cherry picked from commit ea8406b7ebe3eb82670ab054b135b70b005b3374)
Signed-off-by: Robert Varga <rovarga@cisco.com>