Cleanup, De-duplicate with upstream nixpkgs

[phaer]

Note: This is based on https://github.com/nix-community/nixos-generators/pull/375, so please begin your review at 1dee94896d61bc31c71bc832523306efd6db5d50 or wait for a merge of the former PR and the following rebase :)

Started to go through the modules and compare them to upstream nixpkgs in order to reuse new upstream modules (linode) and remove attributes which are nowadays a default with the same priority in upstream nixpkgs.