[SPARK-23956][YARN] Use effective RPC port in AM registration

## What changes were proposed in this pull request?

We propose not to hard-code the RPC port in the AM registration.

## How was this patch tested?

Tested application reports from a pseudo-distributed cluster
```
18/04/10 14:56:21 INFO Client:
client token: N/A
diagnostics: N/A
ApplicationMaster host: localhost
ApplicationMaster RPC port: 58338
queue: default
start time: 1523397373659
final status: UNDEFINED
tracking URL: http://localhost:8088/proxy/application_1523370127531_0016/
```

Author: Gera Shegalov <[email protected]>

Closes apache#21047 from gerashegalov/gera/am-to-rm-nmhost.

Author: gerashegalov

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnRMClient.scala

@@ -29,7 +29,6 @@ import org.apache.spark.{SecurityManager, SparkConf}
 import org.apache.spark.deploy.yarn.config._
 import org.apache.spark.internal.Logging
 import org.apache.spark.rpc.RpcEndpointRef
-import org.apache.spark.util.Utils
 
 /**
  * Handles registering and unregistering the application with the YARN ResourceManager.
@@ -71,7 +70,8 @@ private[spark] class YarnRMClient extends Logging {
 
     logInfo("Registering the ApplicationMaster")
     synchronized {
-      amClient.registerApplicationMaster(Utils.localHostName(), 0, trackingUrl)
+      amClient.registerApplicationMaster(driverRef.address.host, driverRef.address.port,
+        trackingUrl)
       registered = true
     }
     new YarnAllocator(driverUrl, driverRef, conf, sparkConf, amClient, getAttemptId(), securityMgr,