rhn: Fix sam_check

Previously the check was based on the return value, this wasn't reliable
as 0 was also returned when the node was registered to RHN or a
Satellite server.
Now the function is looking for a magic string to determin if SAM is
used or not.

Change-Id: Ifebca13acc78d143ff441076cd9dc0b72766503d
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>