Drop support for `templates`

[grahamc]

Templates are a sort of vestigial feature from who knows when. NixOps' code requires templates live in NixOps itself, and NixOps has no templates already.

[adisbladis]

Something worth considering is using the NixOS module system for evaluating NixOps configurations too.

That way it's easy to make modules that can be imported using the same imports = [ ./some-module.nix ]; syntax as we do in NixOS.