Move from OSTree to OCI for updates of new systems

[jlebon]

Currently, FCOS pushes updates via an OSTree repo. To better align with the bootable containers initiative, let's move to updating FCOS via a container image. Container images are already being published in https://quay.io/repository/fedora/fedora-coreos.

Note this ticket is separate from #1263, which covers also fleshing out the story of layering. In this ticket, we're strictly scoping the effort to changing the transport used.