[SPARK-21124][UI] Show correct application user in UI.

The jobs page currently shows the application user, but it assumes
the OS user is the same as the user running the application, which
may not be true in all scenarios (e.g., kerberos). While it might be
useful to show both in the UI, this change just chooses the application
user over the OS user, since the latter can be found in the environment
page if needed.

Tested in live application and in history server.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #18331 from vanzin/SPARK-21124.

core/src/main/scala/org/apache/spark/ui/SparkUI.scala

@@ -86,7 +86,9 @@ private[spark] class SparkUI private (
   initialize()
 
   def getSparkUser: String = {
-    environmentListener.systemProperties.toMap.getOrElse("user.name", "<unknown>")
+    environmentListener.sparkUser
+      .orElse(environmentListener.systemProperties.toMap.get("user.name"))
+      .getOrElse("<unknown>")
   }
 
   def getAppName: String = appName

core/src/main/scala/org/apache/spark/ui/env/EnvironmentTab.scala

@@ -34,11 +34,16 @@ private[ui] class EnvironmentTab(parent: SparkUI) extends SparkUITab(parent, "en
 @DeveloperApi
 @deprecated("This class will be removed in a future release.", "2.2.0")
 class EnvironmentListener extends SparkListener {
+  var sparkUser: Option[String] = None
   var jvmInformation = Seq[(String, String)]()
   var sparkProperties = Seq[(String, String)]()
   var systemProperties = Seq[(String, String)]()
   var classpathEntries = Seq[(String, String)]()
 
+  override def onApplicationStart(event: SparkListenerApplicationStart): Unit = {
+    sparkUser = Some(event.sparkUser)
+  }
+
   override def onEnvironmentUpdate(environmentUpdate: SparkListenerEnvironmentUpdate) {
     synchronized {
       val environmentDetails = environmentUpdate.environmentDetails