Evidently the readme file hasn't been updated since rhel7 finished
beta, so this is long overdue.
In addition, since it's not possible to download the base image
file directly, let's stop pretending we can and bail out if the user
didn't set the necessary env vars.
Also updated the README to use the new table format instead of free text
Co-Authored-By: Augustina Ragwitz <aragwitz+lp@pobox.com>
Change-Id: Ie8343ee2ce1715583c28de7f59daed7e58c8ca0f