Extracting common functionality for rpm based distros

Rather than dublicating code to implement rhel or any
other derivitive, this patch introduces an rpm-distro
element that should be used as a dependency.

Change-Id: I8a92bb041764d03f430b438f0013704f79a8674c
Remove a device mapping, then let the loop device get removed

Change-Id: Ie1eb621ff56688208421d0115547f127a3c7fd56
Merge "Remove a device mapping, then let the loop device get removed"
Call sync before unmounting keeps the mount from being busy.

here is a bug associated with this patch because there is a
workaround included. Calling sync to enable the unmounting...
https://bugs.launchpad.net/diskimage-builder/+bug/1214388

Change-Id: I9d5a1f092fd8f3ae1c14de03dd516203baab4da3