[SPARK-18417][YARN] Define 'spark.yarn.am.port' in yarn config object

## What changes were proposed in this pull request?
This PR is to define 'spark.yarn.am.port' in yarn config.scala just like other Yarn configurations. That makes code easier to maintain.

## How was this patch tested?
Build passed & tested some Yarn unit tests.

Author: Weiqing Yang <yangweiqing001@gmail.com>

Closes #15858 from weiqingy/yarn.

yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -429,7 +429,7 @@ private[spark] class ApplicationMaster(
   }
 
   private def runExecutorLauncher(securityMgr: SecurityManager): Unit = {
-    val port = sparkConf.getInt("spark.yarn.am.port", 0)
+    val port = sparkConf.get(AM_PORT)
     rpcEnv = RpcEnv.create("sparkYarnAM", Utils.localHostName, port, sparkConf, securityMgr,
       clientMode = true)
     val driverRef = waitForSparkDriver()

yarn/src/main/scala/org/apache/spark/deploy/yarn/config.scala

@@ -40,6 +40,11 @@ package object config {
       .timeConf(TimeUnit.MILLISECONDS)
       .createOptional
 
+  private[spark] val AM_PORT =
+    ConfigBuilder("spark.yarn.am.port")
+      .intConf
+      .createWithDefault(0)
+
   private[spark] val EXECUTOR_ATTEMPT_FAILURE_VALIDITY_INTERVAL_MS =
     ConfigBuilder("spark.yarn.executor.failuresValidityInterval")
       .doc("Interval after which Executor failures will be considered independent and not " +