Added a basic check for the validity of an existing registration via rhn-channel -b

We found that when a system is deleted from spacewalk, this puppet module does not detect that failure and re-register it. We added a simple check (base channel) to the exists? method to allow this to be validated. This has only been tested on our local environment (CentOS 6), but we believe it should generally work. We also moved the "force" check above the logic to test for a systemid file, and made it return false, which is more correct than executing register during the exists? check in our opinion.

~tommy