[SPARK-10549] scala 2.11 spark on yarn with security - Repl doesn't work

Make this lazy so that it can set the yarn mode before creating the securityManager.

Author: Tom Graves <tgraves@yahoo-inc.com>
Author: Thomas Graves <tgraves@staydecay.corp.gq1.yahoo.com>

Closes #8719 from tgravescs/SPARK-10549.

repl/scala-2.11/src/main/scala/org/apache/spark/repl/Main.scala

@@ -35,7 +35,8 @@ object Main extends Logging {
   s.processArguments(List("-Yrepl-class-based",
     "-Yrepl-outdir", s"${outputDir.getAbsolutePath}",
     "-classpath", getAddedJars.mkString(File.pathSeparator)), true)
-  val classServer = new HttpServer(conf, outputDir, new SecurityManager(conf))
+  // the creation of SecurityManager has to be lazy so SPARK_YARN_MODE is set if needed
+  lazy val classServer = new HttpServer(conf, outputDir, new SecurityManager(conf))
   var sparkContext: SparkContext = _
   var sqlContext: SQLContext = _
   var interp = new SparkILoop // this is a public var because tests reset it.