Add rhsm_subscription_type fact

[kallies]

Fixes #64

[kallies]

Discussion with @vinzent about the usefulness of the fact. The idea is that it is possible to check if the system is still subscribed to RHN classic (i.e. systemid file exists). If this is the case subscription-manager register will output a warning that the system is already registered. It is recommended that the system is only registered once.

Caveat:

• the system is still registered on the RHN Classic/RH Satellite 5, even though the system is not aware if this fact. Cleaning up RHN is not in scope of this module
• there are other possible leftovers from RHN Classic registration (e.g. osad, *rhn* and *spacewalk*)