Fixes #3842: add support for registering with RHSM

This pull request introduces support for registering with subscription-manager without changing any of the current behavior around `rhnreg_ks`. I believe I've tested each case (hosted with rhnreg_ks, standalone spacewalk with rhnreg_ks, subscription-manager with RHN, and subscription-manager with Katello/SAM), but some more testing would be good.