Adding rhel installation element

-RHEL support for partitioning
 RHEL 6 doesn't create /dev/loop*p* devices for partitions. It does
 use device mapper to create the device under /dev/mapper/loop*p*.
-Removing the -r (read-only) switch for kpartx.
-Although it's safer, -r isn't an option on a rhel host version of kpartx.
-Fixing an unmount issue on rhel

Change-Id: Idb547f4ffe75514b1e3f6b34f5f347493b132925