MachineState: Don't send keys within state.RESCUE.

This will store keys on the machine's disk when you try to deploy a
machine expression that has deployment.keys set, because in rescue state
we don't have /run mounted.

If you want to manually get rid of /run/keys from your disk you can
access the files that are covered by another mount using a bind mount of
the parent directory, so you can delete the keys using something like
this:

mkdir /delkeys
mount --bind / /delkeys
rm -r /delkeys/run/keys
umount /delkeys
rmdir /delkeys

But that's no longer necessary since release 14.04, because the stage 2
init script does a "rm -rf /run/keys" on bootup already.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>