Add simple Registry class to manage singletons

Add simple Registry class to manage singletons

Typically, singleton classes have lots of static methods, or use
a getInstance() method to get the singleton.  This provides an alternative
mechanism, similar to the JDNI and the common-paramater property registry,
where singletons can be registered.

Clean up registry after junit test.
Modified a few comments. (I prefer 90 characters for comments.)

Added a method to register or replace an existing key without throwing
an exception.

Change-Id: I3b62719013d3b5f71adb5e9299d3c1257fb55c80
Issue-ID: POLICY-1542
Signed-off-by: Jim Hahn <jrh3@att.com>