core: Adding getRpmRevision to RpmVersion

RpmVersion should be improved and rewritten in such
a way that frontend and engine will not need to use two classes.
However, for now an interim version is presented - calculation of the
RpmRevision part - RpmRevision part can be in format of a colon
delimited string, and has nothing to do with the revision field of
Version (in the future we will also change the inheritance)

Change-Id: I52499df93cd7cadc6efb6c49484ea95b1c95fb59
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>