Move Virtualbox base image to nixpkgs for quicker iterations

[domenkozar]

We've done the same before for Amazon.

[davidak]

I followed the manual and 18.03pre was used...

[DavHau]

I opened a PR for nixpkgs: https://github.com/NixOS/nixpkgs/pull/97335 Maybe you could check if it is done the right way.

With this change, the base image can be referenced via: (import "${pkgs.path}/nixos/release.nix" {}).nixopsVboxImage;

If the PR gets merged, I will try to add this to nixops.