Motivation for this change

NixOps' virtualbox module has the problem to constantly depend on an outdated base image of NixOS.
The following issues are most likely a consequence of this:
NixOS/nixops#1207
NixOS/nixops#835
NixOS/nixops#908

The general problem seems to be that the base images are manually built by maintainers and hardcoded in the nixops vbox module.
Therefore, by design, any stable nixops version found in nixpkgs can never refer to an up-to-date base image of the same nixpkgs version.

Things done

Following the suggestion from nix-community/nixops-vbox#9, I'd like to add the image to nixpkgs which will allow nixops users to always get a recent base image from the nixos cache.
Future versions of nixops could just point to this nixpkgs attribute and allow for an improved user experience.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.