nixos/storage: Add defaults for clear/initlabel

I haven't yet stumbled on this because I have always set these options
within test configurations.

So this now allows to set empty disk options, which is fine (after all
we just need to reference them using the storage option in fileSystems
and swapDevices).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/tasks/storage.nix

@@ -116,13 +116,15 @@ let
   diskOptions.options = {
     clear = mkOption {
       type = types.bool;
+      default = false;
       description = ''
         Clear the partition table of this device.
       '';
     };
 
     initlabel = mkOption {
       type = types.bool;
+      default = false;
       description = ''
         Create a new disk label for this device (implies
         <option>clear</option>).