SSH known_hosts is not maintained by Hetzner and none backends

[roberth]

Most backends store the ssh host key in the state file, so it is sufficient to only share the state file. If you have a hetzner or none deployment this doesn't happen, putting the burden on the user.

I've noticed that the implementations that do implement it each have their own field for this purpose. Perhaps this could be unified? For example, here's how nixops-aws uses ec2.publicHostKey.

public_host_key = nixops.util.attr_property("ec2.publicHostKey", None)

I'm ok with, say, none.publicHostKey and hetzner.publicHostKey though.