hydra: only set buildMachinesFiles when nix.buildMachines is defined

This applies [hydra PR #432](https://github.com/NixOS/hydra/pull/432)
to the NixOS module in nixpkgs:

```
commit 4efd078977e5ea20e1104783efc324cba11690bc
Author: Bas van Dijk <v.dijk.bas@gmail.com>
Date:   Sun Dec 11 15:35:38 2016 +0100

    Only set buildMachinesFiles when nix.buildMachines is defined
```

nixos/modules/services/continuous-integration/hydra/default.nix

@@ -167,7 +167,7 @@ in
 
       buildMachinesFiles = mkOption {
         type = types.listOf types.path;
-        default = [ "/etc/nix/machines" ];
+        default = optional (config.nix.buildMachines != []) "/etc/nix/machines";
         example = [ "/etc/nix/machines" "/var/lib/hydra/provisioner/machines" ];
         description = "List of files containing build machines.";
       };