cluster/nix: integrate with readtree

This unifies nixpkgs with the one defined in //default.nix and makes it
possible to use readTree to build the provisioners:

   nix-build -A cluster.nix.provision

   result/bin/provision

Change-Id: I68dd70b9c8869c7c0b59f5007981eac03667b862
diff --git a/cluster/clustercfg/clustercfg.py b/cluster/clustercfg/clustercfg.py
index c5f5c6c..0adef40 100644
--- a/cluster/clustercfg/clustercfg.py
+++ b/cluster/clustercfg/clustercfg.py
@@ -207,8 +207,8 @@
         ca_admitomatic.make_cert('admitomatic-webhook', ou='Admitomatic Webhook', hosts=['admitomatic.admitomatic.svc'])
 
     subprocess.check_call(["nix", "run",
-                           "-f", os.path.join(local_root, "cluster/nix/default.nix"),
-                           "provision",
+                           "-f", local_root,
+                           "cluster.nix.provision",
                            "-c", "provision-{}".format(fqdn.split('.')[0])])