Atomic can be provisioned now from upstream entirely, removing the need for this parameter. Before http://projects.theforeman.org/issues/13340 , we had to set up a local installation media for the PXE boot stuff. So provisioning from upstream was optional. Now, we can default to provision from upstream and anyone with a custom installation media can change the kickstart to use it.

CentOS unfortunately still has to be provisioned through a local installation media so I'm not changing that line.