nixos/systemd-nspawn: disallow multiple packages with .nspawn-units

In contrast to `.service`-units, it's not possible to declare an
`overrides.conf`, however this is done by `generateUnits` for `.nspawn`
units as well. This change breaks the build if you have two derivations
configuring one nspawn unit.

This will happen in a case like this:

``` nix
{ pkgs, ... }: {
  systemd.packages = [
    (pkgs.writeTextDir "etc/systemd/nspawn/container0.nspawn" ''
      [Files]
      Bind=/tmp
    '')
  ];
  systemd.nspawn.container0 = {
    /* ... */
  };
}
```

nixos/modules/system/boot/systemd-nspawn.nix

@@ -116,7 +116,7 @@ in {
     in 
       mkMerge [
         (mkIf (cfg != {}) { 
-          environment.etc."systemd/nspawn".source = mkIf (cfg != {}) (generateUnits "nspawn" units [] []);
+          environment.etc."systemd/nspawn".source = mkIf (cfg != {}) (generateUnits' false "nspawn" units [] []);
         })
         {
           systemd.targets.multi-user.wants = [ "machines.target" ];