Use debug logging rather than info for rejected ping task

This exception is thrown on node shutdown and doesn't indicate
an critical situation but rather is caught for consistency reasons.

src/main/java/org/elasticsearch/discovery/zen/ping/unicast/UnicastZenPing.java

@@ -185,7 +185,7 @@ public class UnicastZenPing extends AbstractLifecycleComponent<ZenPing> implemen
                             }
                             listener.onPing(responses.values().toArray(new PingResponse[responses.size()]));
                         } catch (RejectedExecutionException ex) {
-                            logger.info("Ping execution ejected", ex);
+                            logger.debug("Ping execution ejected", ex);
                         }
                     }
                 });