[Discovery] Start master fault detection after pingInterval

This is to allow the master election to complete on the chosen master.

 Relates to #6706

src/main/java/org/elasticsearch/discovery/zen/fd/MasterFaultDetection.java

@@ -156,8 +156,8 @@ public class MasterFaultDetection extends AbstractComponent {
         }
         this.masterPinger = new MasterPinger();
 
-        // we use schedule with a 0 time value to run the pinger on the pool as it will run on later
+        // we start pinging slightly later to allow the chosen master to complete it's own master election
-        threadPool.schedule(TimeValue.timeValueMillis(0), ThreadPool.Names.SAME, masterPinger);
+        threadPool.schedule(pingInterval, ThreadPool.Names.SAME, masterPinger);
     }
 
     public void stop(String reason) {