Options json

This add a derivation which would be used by hydra and can be used locally for improving our tools around NixOS.  A tool can get the list of option as follow:

```
$ cd .../nixos
$ nix-build -A config.system.build.manual.options -o options
$ ls options/share/doc/nixos/
options.json  options.xml
```

Maybe it was intended as a separate commit.