[SPARK-6769][YARN][TEST] Usage of the ListenerBus in YarnClusterSuite is wrong

In YarnClusterSuite, a test case uses `SaveExecutorInfo`  to handle ExecutorAddedEvent as follows.

```
private class SaveExecutorInfo extends SparkListener {
  val addedExecutorInfos = mutable.Map[String, ExecutorInfo]()

  override def onExecutorAdded(executor: SparkListenerExecutorAdded) {
    addedExecutorInfos(executor.executorId) = executor.executorInfo
  }
}

...

    listener = new SaveExecutorInfo
    val sc = new SparkContext(new SparkConf()
      .setAppName("yarn \"test app\" 'with quotes' and \\back\\slashes and $dollarSigns"))
    sc.addSparkListener(listener)
    val status = new File(args(0))
    var result = "failure"
    try {
      val data = sc.parallelize(1 to 4, 4).collect().toSet
      assert(sc.listenerBus.waitUntilEmpty(WAIT_TIMEOUT_MILLIS))
      data should be (Set(1, 2, 3, 4))
      result = "success"
    } finally {
      sc.stop()
      Files.write(result, status, UTF_8)
    }
```

But, the usage is wrong because Executors will spawn during initializing SparkContext and SparkContext#addSparkListener should be invoked after the initialization, thus after Executors spawn, so SaveExecutorInfo cannot handle ExecutorAddedEvent.

Following code refers the result of the handling ExecutorAddedEvent. Because of the reason above, we cannot reach the assertion.

```
    // verify log urls are present
    listener.addedExecutorInfos.values.foreach { info =>
      assert(info.logUrlMap.nonEmpty)
    }
```

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #5417 from sarutak/SPARK-6769 and squashes the following commits:

8adc8ba [Kousuke Saruta] Fixed compile error
e258530 [Kousuke Saruta] Fixed style
591cf3e [Kousuke Saruta] Fixed style
48ec89a [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-6769
860c965 [Kousuke Saruta] Simplified code
207d325 [Kousuke Saruta] Added findListenersByClass method to ListenerBus
2408c84 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-6769
2d7e409 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-6769
3874adf [Kousuke Saruta] Fixed the usage of listener bus in LogUrlsStandaloneSuite
153a91b [Kousuke Saruta] Fixed the usage of listener bus in YarnClusterSuite