Avoid the target downloading from cache but copy everything.

[ambrop72]

I don't like how when I deploy to a local machine, even though the nixops host has the whole system built locally (which is for example ensured by using deploy --build-only), when I deploy, the target system downloads many things from the public cache. I would like an option in nixops to stop the target system downloading anything from the cache but copy anything missing from the nixops host.

[domenkozar]

def has_really_fast_connection(self): is responsible for that atm.

[danbst]

related #513

[domenkozar]

@danbst #566