Motivation for this change

As it says.

For instance, I have a piece of code that creates a large number of VMs, which differ only in their networking.hostName.

While wrapping 'configuration' does work, adding this parameter vastly simplifies said code, while minimizing coupling; it reduces to nixos { configuration = ./base-node.nix; modules = [{ networking.hostName = hostName; }]; }.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.