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.

Change-Id: Idb547f4ffe75514b1e3f6b34f5f347493b132925