[SPARK-3777] Display "Executor ID" for Tasks in Stage page

Now the Stage page only displays "Executor"(host) for tasks. However, there may be more than one Executors running in the same host. Currently, when some task is hung, I only know the host of the faulty executor. Therefore I have to check all executors in the host.

Adding "Executor ID" in the Tasks table. would be helpful to locate the faulty executor. Here is the new page:

![add_executor_id_for_tasks](https://cloud.githubusercontent.com/assets/1000778/4505774/acb9648c-4afa-11e4-8826-8768a0a60cc9.png)

Author: zsxwing <[email protected]>

Closes apache#2642 from zsxwing/SPARK-3777 and squashes the following commits:

37945af [zsxwing] Put Executor ID and Host into one cell
4bbe2c7 [zsxwing] [SPARK-3777] Display "Executor ID" for Tasks in Stage page

Author: zsxwing

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -103,7 +103,7 @@ private[ui] class StagePage(parent: JobProgressTab) extends WebUIPage("stage") {
 
       val taskHeaders: Seq[String] =
         Seq(
-          "Index", "ID", "Attempt", "Status", "Locality Level", "Executor",
+          "Index", "ID", "Attempt", "Status", "Locality Level", "Executor ID / Host",
           "Launch Time", "Duration", "GC Time", "Accumulators") ++
         {if (hasInput) Seq("Input") else Nil} ++
         {if (hasShuffleRead) Seq("Shuffle Read")  else Nil} ++
@@ -282,7 +282,7 @@ private[ui] class StagePage(parent: JobProgressTab) extends WebUIPage("stage") {
         }</td>
         <td>{info.status}</td>
         <td>{info.taskLocality}</td>
-        <td>{info.host}</td>
+        <td>{info.executorId} / {info.host}</td>
         <td>{UIUtils.formatDate(new Date(info.launchTime))}</td>
         <td sorttable_customkey={duration.toString}>
           {formatDuration}