1128658: do not contact RHN if unregistered

when running 'yum --disablerepo=\* makecache' we shouldn't attempt to contact
RHN. We were always making a call to get overrides which doesn't make sense if
we are not registered.
Merge pull request #1018 from candlepin/bz1128658

1128658: do not contact RHN if unregistered