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. Also since
the interaction between path and mount units can lead to unexpected
behavior if something goes wrong the cloudinit service is now triggered
explicitly by the mount again. The configdrive path unit remains only as
a fall back for containers where the mount unit doesn't kick in. Better
to have two mechanisms that trigger the cloudinit service than none. :)

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.