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.

TEST
--------
* install this patched version of subsription-manager on RHEL 6.6
* install ```wireshark```
* open 2 terminals 
* in one terminal run wireshark: ```tshark -f 'dst host subscription.rhn.redhat.com'```
* in the other run ```yum --disablerepo=\* makecache```
* There should be NO traffic in the wireshark terminal.