clustercfg: show diff before switching to new configuration

This is mildly hacky, but lets us be more informed before we switch to a
new configuration.

Change-Id: I008f3f698db702f1e0992bd41a8d1050449d59b5
diff --git a/cluster/clustercfg/clustercfg.py b/cluster/clustercfg/clustercfg.py
index 1396826..410635b 100644
--- a/cluster/clustercfg/clustercfg.py
+++ b/cluster/clustercfg/clustercfg.py
@@ -205,8 +205,7 @@
     subprocess.check_call(["nix", "run",
                            "-f", os.path.join(local_root, "cluster/nix/default.nix"),
                            "provision",
-                           "-c", "provision-{}".format(fqdn.split('.')[0]),
-                           "switch"])
+                           "-c", "provision-{}".format(fqdn.split('.')[0])])
 
 
 def usage():