HDFS-12347. TestBalancerRPCDelay#testBalancerRPCDelay fails very frequently. Contributed by Bharat Viswanadham

(cherry picked from commit c7499f2d242c64bee8f822a22161d956525f7153)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/balancer/TestBalancer.java

@@ -1966,7 +1966,7 @@ public class TestBalancer {
     initConf(conf);
     conf.setInt(DFSConfigKeys.DFS_BALANCER_DISPATCHERTHREADS_KEY, 30);
 
-    int numDNs = 40;
+    int numDNs = 20;
     long[] capacities = new long[numDNs];
     String[] racks = new String[numDNs];
     for(int i = 0; i < numDNs; i++) {