[SPARK-1631] Correctly set the Yarn app name when launching the AM.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #539 from vanzin/yarn-app-name and squashes the following commits:

7d1ca4f [Marcelo Vanzin] [SPARK-1631] Correctly set the Yarn app name when launching the AM.

yarn/common/src/main/scala/org/apache/spark/scheduler/cluster/YarnClientSchedulerBackend.scala

@@ -35,10 +35,10 @@ private[spark] class YarnClientSchedulerBackend(
 
   private[spark] def addArg(optionName: String, envVar: String, sysProp: String,
       arrayBuf: ArrayBuffer[String]) {
-    if (System.getProperty(sysProp) != null) {
-      arrayBuf += (optionName, System.getProperty(sysProp))
-    } else if (System.getenv(envVar) != null) {
+    if (System.getenv(envVar) != null) {
       arrayBuf += (optionName, System.getenv(envVar))
+    } else if (sc.getConf.contains(sysProp)) {
+      arrayBuf += (optionName, sc.getConf.get(sysProp))
     }
   }