Add installation machine

Its configuration does not fit a real machine, but rather serves as a
minimal configuration for new machines during installation.

machines/installation/configuration.nix

@@ -0,0 +1,21 @@
+{ config, pkgs, ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+  ];
+
+  networking.useDHCP = false;
+  # FIXME: replace with actual network interface
+  networking.interfaces.eno1.useDHCP = true;
+
+  environment.systemPackages = with pkgs; [
+    git # required by krops
+  ];
+
+  services.openssh.enable = true;
+
+  users.users.root.openssh.authorizedKeys.keys = [
+    # FIXME: add ssh keys required for initial deployment with krops
+  ];
+}