[Nix-dev] Enable openntpd instead ntp by default

[Peter Simons]

Hi William,

 > Feel free to try enabling timesyncd by simply setting
 > `services.timesyncd.enable` in your nixos config. It should work as a drop
 > in replacement and respect the servers set in `services.ntp. servers`.

I tried that on a system running the current "unstable" channel, but the
service fails an assertion:

  https://github.com/NixOS/nixpkgs/issues/5913

Best regards,
Peter