Update the image name for RHEL Guest Image

The RHEL 6 Guest Image has been updated with a new release that
addresses a number of bugs.  The diskimage-builder element that
references this file needed to be updated to reference the correct
image name.  In addition, some notes to help users download the
image from RHN have been added to the README.

Change-Id: I3c9e64a1887afd9c0b370d70eff4b7f9598cebd1
Closes-bug: launchpad-bug-1286568