[jvm-packages] RabitTracker for Scala: allow specifying host ip from …

…the xgboost-tracker.properties file (dmlc#3833)

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/rabit/RabitTracker.scala

@@ -20,7 +20,7 @@ import java.net.{InetAddress, InetSocketAddress}
 
 import akka.actor.ActorSystem
 import akka.pattern.ask
-import ml.dmlc.xgboost4j.java.IRabitTracker
+import ml.dmlc.xgboost4j.java.{IRabitTracker, TrackerProperties}
 import ml.dmlc.xgboost4j.scala.rabit.handler.RabitTrackerHandler
 
 import scala.concurrent.duration._
@@ -93,8 +93,11 @@ private[scala] class RabitTracker(numWorkers: Int, port: Option[Int] = None,
     * @return Boolean flag indicating if the Rabit tracker starts successfully.
     */
   private def start(timeout: Duration): Boolean = {
+    val hostAddress = Option(TrackerProperties.getInstance().getHostIp)
+      .map(InetAddress.getByName).getOrElse(InetAddress.getLocalHost)
+
     handler ? RabitTrackerHandler.StartTracker(
-      new InetSocketAddress(InetAddress.getLocalHost, port.getOrElse(0)), maxPortTrials, timeout)
+      new InetSocketAddress(hostAddress, port.getOrElse(0)), maxPortTrials, timeout)
 
     // block by waiting for the actor to bind to a port
     Try(Await.result(handler ? RabitTrackerHandler.RequestBoundFuture, askTimeout.duration)