HADOOP-12993. Change ShutdownHookManger complete shutdown log from INFO to DEBUG. Contributed by Xiaoyu Yao.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/ShutdownHookManager.java

@@ -81,7 +81,7 @@ public class ShutdownHookManager {
                 LOG.error("ShutdownHookManger shutdown forcefully.");
                 EXECUTOR.shutdownNow();
               }
-              LOG.info("ShutdownHookManger complete shutdown.");
+              LOG.debug("ShutdownHookManger complete shutdown.");
             } catch (InterruptedException ex) {
               LOG.error("ShutdownHookManger interrupted while waiting for " +
                   "termination.", ex);