Add rhnreg_ks module

 The rhnreg_ks module acts as a wrapper to the command 'rhnreg_ks'. The rhnreg_ks command allows users to manage registration to a Red Hat Network (RHN) (e.g. rhn.redhat.com) server.  The moduel will also interact with the specified RHN system via XMLRPC as needed.

Before proceeding with registration, the rhnreg_ks module will enable the system to receive updates from RHN.  This involves enabling the appropriate RHN yum plugin, as well as disabling an active subscription-manager yum plugin.

Once enabled, the module will support the following operations:
 * configure
 * register
 * subscribe to custom child channels (see `rhn-channel` command)
 * unregister

This pull-request includes some adjustments to the previous subscription-manager module.  Given the changes are related to rhnreg_ks, I've included them in this pull-request.  I can easily split this into a different pull request, if desired.