README.md: Undeprecate now that we have imagefactory here too

While we kept the old create-vm-disk code, there is now 'imagefactory'
as a builtin.
lorax: Also template OS name (don't hardcode Fedora)

This way the tooling can be more easily used by downstreams.
create-vm-disk: Support writing an unconfigured-state message

See https://github.com/projectatomic/rpm-ostree/issues/31