nixos/nova-image: cleanup image builders (#29242)

There are currently two ways to build Openstack image. This just picks
best of both, to keep only one!

- Image is resizable
- Cloudinit is enable
- Password authentication is disable by default
- Use the same layer than other image builders (ec2, gce...)

nixos/maintainers/scripts/openstack/nova-image.nix

···
+# nix-build '<nixpkgs/nixos>' -A config.system.build.novaImage --arg configuration "{ imports = [ ./nixos/maintainers/scripts/openstack/nova-image.nix ]; }"
+
 { config, lib, pkgs, ... }:
 
 with lib;

nixos/modules/virtualisation/cloud-image.nix

···
-# Usage:
-# $ NIX_PATH=`pwd`:nixos-config=`pwd`/nixpkgs/nixos/modules/virtualisation/cloud-image.nix nix-build '<nixpkgs/nixos>' -A config.system.build.cloudImage
-
-{ config, lib, pkgs, ... }:
-
-with lib;
-
-{
-  system.build.cloudImage = import ../../lib/make-disk-image.nix {
-    inherit pkgs lib config;
-    partitioned = true;
-    diskSize = 1 * 1024;
-    configFile = pkgs.writeText "configuration.nix"
-      ''
-        { config, lib, pkgs, ... }:
-
-        with lib;
-
-        {
-          imports = [ <nixpkgs/nixos/modules/virtualisation/cloud-image.nix> ];
-        }
-      '';
-  };
-
-  imports = [ ../profiles/qemu-guest.nix ];
-
-  fileSystems."/".device = "/dev/disk/by-label/nixos";
-
-  boot = {
-    kernelParams = [ "console=ttyS0" ];
-    loader.grub.device = "/dev/vda";
-    loader.timeout = 0;
-  };
-
-  networking.hostName = mkDefault "";
-
-  services.openssh = {
-    enable = true;
-    permitRootLogin = "without-password";
-    passwordAuthentication = mkDefault false;
-  };
-
-  services.cloud-init.enable = true;
-}

nixos/modules/virtualisation/nova-config.nix

···
     boot.loader.timeout = 0;
 
     # Allow root logins
-    services.openssh.enable = true;
-    services.openssh.permitRootLogin = "prohibit-password";
+    services.openssh = {
+      enable = true;
+      permitRootLogin = "prohibit-password";
+      passwordAuthentication = mkDefault false;
+    };
+
+    services.cloud-init.enable = true;
 
     # Put /tmp and /var on /ephemeral0, which has a lot more space.
     # Unfortunately we can't do this with the `fileSystems' option