Fix an unsafe formatting bug

This might fix issue 84; in any case, the current situation is that we
have a potentially unsafe formatting, which should be fixed.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/mcpu.py

@@ -342,8 +342,7 @@ class HooksMaster(object):
         res = results[node_name]
         if res.failed or res.data is False or not isinstance(res.data, list):
           if not res.offline:
-            self.proc.LogWarning("Communication failure to node %s" %
-                                 node_name)
+            self.proc.LogWarning("Communication failure to node %s", node_name)
           continue
         for script, hkr, output in res.data:
           if hkr == constants.HKR_FAIL: