1. [WIP] core: Changing UserDomainsCacheManager to be a regular singleton

There is no particular need for this to be an EJB singleton.

Change-Id: I68d4cff4ecf7bbf0052d0473f9a833e17b4e8d1c
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>