deployment: Fix hard reset for reboot to rescue.

This is particularly annoying if you have a dead server and want to put
it in rescue, so in order to do that you either needed to change the
deyloyment state or reboot manually into rescue using the robot
interface or something like hetznerctl.

Now it's possible to do that simply by:

nixops reboot --hard --rescue --include=deadmachine

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hetzner: Always hard reset on destroying machine.

We're going to kill off the machine anyway, so why should we try to send
a reboot command using SSH?

Especially useful for getting rid of dead machines.

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