fix(configdrive): Use mount units, give virtfs a new mount point.

Currently systemd cannot track dependencies on configdrive very well
because it is mounted via a service instead of a mount unit.

Since mounting a virtfs based configdrive requires different mount
options and two different mount units cannot refer to the same path the
virtfs version now mounts to /media/configvirtfs.

There are also two new kernel options:
- `coreos.configdrive=1`: enable config drive on physical hardware.
- `coreos.configdrive=0`: disable config drive on virtual machines.