Fixes #13340 - Seed Fedora Atomic installation media

Fedora Atomic now has a repository where you can get the isolinux stuff
from -https://dl.fedoraproject.org/pub/alt/atomic/stable/Cloud_Atomic/x86_64/os/

This allows us to perform fully unattended installations through
foreman. Before, you would have to download the image and prepare your
own local installation media to PXE boot it
(http://blog.daniellobato.me/unattended-deployments-of-fedora-and-rhel-atomic-with-foreman/).