This will break 16.03

Oh, sorry, didn't think about that.

We can merge once 16.09 is released.

Actually, there are more of this:

nixops/nix/libvirtd-image.nix:      boot.loader.grub.timeout = 0;
nixops/nix/hetzner.nix:    boot.loader.grub.timeout = 1;
nixops/nix/virtualbox-image-nixops.nix:  boot.loader.grub.timeout = 1;
nixops/nix/libvirtd.nix:    boot.loader.grub.timeout = 0;

Changing these is problematic because it means we won't be able to deploy older NixOS versions anymore. I guess we need some way to make option definitions conditional on the NixOS version. However, the module system AFAIK doesn't allow that directly. E.g. you can't say

config = mkIf (... version >= 16.09 ...) {
  boot.loader.timeout = 1;
};

because the options under a mkIf still need to exist even if their value is not used.

As a compromise, would it be viable/possible to add boot.loader.timeout as an alias option for older branches of Nixpkgs?

That way, you'd still be unable to deploy older builds of those branches, but if they're pegging Nixpkgs to a specific commit then it shouldn't be too big of a deal to peg Nixops as well.

Yeah, that would be fine.

This exact occurance was already fixed by @rbvermaa and the code already does the correct thing in 16.03, so I'll close this and fix the remaining occurances.