sshd: Support multiple host keys

The option services.openssh.hostKeys now allows specifying multiple
host keys.  The default value enables both a DSA and ECDSA key.
(Clients by default will use the ECDSA key, unless known_hosts already
has a DSA key for that host.)  To use only an ECDSA key, you can say:

  services.openssh.hostKeys =
    [ { path = "/etc/ssh/ssh_host_ecdsa_key";
        type = "ecdsa";
        bits = 521;
      }
    ];