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

@@ -1,3 +1,5 @@
+# nix-build '<nixpkgs/nixos>' -A config.system.build.novaImage --arg configuration "{ imports = [ ./nixos/maintainers/scripts/openstack/nova-image.nix ]; }"
+
 { config, lib, pkgs, ... }:
 
 with lib;