HDFS-10963. Reduce log level when network topology cannot find enough datanodes. Contributed by Xiao chen

(cherry picked from commit b90fc70d67)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/NetworkTopology.java

@@ -763,7 +763,7 @@ public class NetworkTopology {
       }
     }
     if (numOfDatanodes == 0) {
-      LOG.warn("Failed to find datanode (scope=\"{}\" excludedScope=\"{}\").",
+      LOG.debug("Failed to find datanode (scope=\"{}\" excludedScope=\"{}\").",
           String.valueOf(scope), String.valueOf(excludedScope));
       return null;
     }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockPlacementPolicyDefault.java

@@ -49,7 +49,8 @@ public class BlockPlacementPolicyDefault extends BlockPlacementPolicy {
 
   private static final String enableDebugLogging =
       "For more information, please enable DEBUG log level on "
-    + BlockPlacementPolicy.class.getName();
+          + BlockPlacementPolicy.class.getName() + " and "
+          + NetworkTopology.class.getName();
 
   private static final ThreadLocal<StringBuilder> debugLoggingBuilder
       = new ThreadLocal<StringBuilder>() {