Allow or document way to specify initial ssh key for none backend.

[andrewchambers]

I am using nixops to manage machines in an automated way, the machines are provisioned outside of nixops, so i need a way to specify the initial ssh key. Using a global ssh config file is not suitable.

I am thinking something like an attribute option to specify to nixops which externally generated ssh keys to use that works across all backends.

[andrewchambers]

Another alternative may be an option such as a "NIXOPS_SSH_CONFIG" environment variable.

[coretemp]

I prefer the attribute version of this.

[andrewchambers]

The option version with the pull request open is modeled after the option to nix-copy-closure.