[SPARK-8136] [YARN] Fix flakiness in YarnClusterSuite.

Instead of actually downloading the logs, just verify that the logs link is actually
a URL and is in the expected format.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #6680 from harishreedharan/simplify-am-log-tests and squashes the following commits:

3183aeb [Hari Shreedharan] Remove check for hostname which can fail on machines with several hostnames. Removed some unused imports.
50d69a7 [Hari Shreedharan] [SPARK-8136][YARN] Fix flakiness in YarnClusterSuite.

yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnClusterSuite.scala

@@ -18,12 +18,12 @@
 package org.apache.spark.deploy.yarn
 
 import java.io.{File, FileOutputStream, OutputStreamWriter}
+import java.net.URL
 import java.util.Properties
 import java.util.concurrent.TimeUnit
 
 import scala.collection.JavaConversions._
 import scala.collection.mutable
-import scala.io.Source
 
 import com.google.common.base.Charsets.UTF_8
 import com.google.common.io.ByteStreams
@@ -344,18 +344,20 @@ private object YarnClusterDriver extends Logging with Matchers {
       assert(info.logUrlMap.nonEmpty)
     }
 
-    // If we are running in yarn-cluster mode, verify that driver logs are downloadable.
+    // If we are running in yarn-cluster mode, verify that driver logs links and present and are
+    // in the expected format.
     if (conf.get("spark.master") == "yarn-cluster") {
       assert(listener.driverLogs.nonEmpty)
       val driverLogs = listener.driverLogs.get
       assert(driverLogs.size === 2)
       assert(driverLogs.containsKey("stderr"))
       assert(driverLogs.containsKey("stdout"))
-      val stderr = driverLogs("stderr") // YARN puts everything in stderr.
-      val lines = Source.fromURL(stderr).getLines()
-      // Look for a line that contains YarnClusterSchedulerBackend, since that is guaranteed in
-      // cluster mode.
-      assert(lines.exists(_.contains("YarnClusterSchedulerBackend")))
+      val urlStr = driverLogs("stderr")
+      // Ensure that this is a valid URL, else this will throw an exception
+      new URL(urlStr)
+      val containerId = YarnSparkHadoopUtil.get.getContainerId
+      val user = Utils.getCurrentUserName()
+      assert(urlStr.endsWith(s"/node/containerlogs/$containerId/$user/stderr?start=0"))
     }
   }