Invert default of config.registration.unregister_on_halt

[hferentschik]

The default of config.registration.unregister_on_halt should be false. It makes more sense to just un-register on destroy. If one really wants to un-register on halt, one can explicitly set it.

[dkutalek]

One possible problem: unregister needs running machine, doesn't it?

Therefore:

• up: register
• halt: leave machine registered, but shut it down
• destroy: will not unregister?

User would have to do up / destroy just to destroy with unregister.