Adding rhel installation element

Adding 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*.

This patch adds a common functioon to figure out what operating
system disk image builder is being run on, then decides where to look
for the loop partition.
Removing the -r (read-only) switch for kpartx.

- Although it's safer, -r isn't an option on a rhel host version of kpartx.