Use dedicated SSH keypair for "none" backend.

This should make the "none" backend a bit more useful as we only need to provide a key _or_ passphrase on machine creation and from thereon we only use the SSH keys within NixOps' state database.

If you have a large set of "none" machines and want to export/import them, this should no longer require to move around `ssh_config`s or even entries in your hosts file.

In addition, this fixes the situation where you need to build on the target machines and the none backend wasn't able to provide a private key for the to be generated `nix.machines` file.

VM test result: https://headcounter.org/hydra/job/nixops/none-improvements/tests.none_backend/latest
(also required fixing the VM tests)

This is related to #200 and after talking to @joelton on IRC it turned out he was on Darwin, so NixOps was trying to build on the target machine(s), which in turn requires explicitly specifying a private key. As we now have the keypair in our own db, this is no longer an issue.
Why not use `users.extraUsers.root.openssh.authorizedKeys` to set the key? `get_physical_spec` can already emit it.