Updated rhel element to support rhn

Added support for Red Hat Network (RHN) subscription. Extended to accept list of rhn channels or repos to enable. Default subscription method remains rhsm for backward compatibility. Also updated the default RHEL image to the latest RHEL 6.5 image available on the Customer Portal.

Change-Id: Ia7f39bee7ae24253fa2002b3dc5b2293be3ff0bd