MachineState: Don't send keys while in 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.

Depending on the use of this, it might lead to serious security
implications, so be sure to (possibly securely) delete /run while in
rescue mode to avoid having those keys lingering around on your disk.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit a8a4f240f3b809720d59b55c3d51840a422efa17)