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 RHSM_USER to your rhn username
export 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
If the loop partition isn't created as a device by the kernel, go
ahead and use kpartx to create the device.
Remove a device mapping, then let the loop device get removed
Call sync before unmounting keeps the mount from being busy.
Kpartx on rhel doesn't have a read only option

- When building a fedora image on a rhel host you can't use a
  read only option because it doesn't exist.