Add rhel installation element

If you want to have the installation update packages, you'll
need to register the system log in to rhn and subscribe to an
available subscription.
export DIB_RHSM_USER to your rhn username
export DIB_RHSM_PASSWORD to your rhn password

To get the qcow2 image, log into rhn.redhat.com and download the
image from
https://rhn.redhat.com/rhn/software/channel/downloads/Download.do?cid=16952
Then export DIB_CLOUD_IMAGES to whereever you're hosting the qcow2.

Change-Id: Idb547f4ffe75514b1e3f6b34f5f347493b132925