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