[SPARK-7162] [YARN] Launcher error in yarn-client

jira: https://issues.apache.org/jira/browse/SPARK-7162

Author: GuoQiang Li <witgo@qq.com>

Closes #5716 from witgo/SPARK-7162 and squashes the following commits:

b64564c [GuoQiang Li] Launcher error in yarn-client

yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -354,7 +354,7 @@ private[spark] class Client(
         val dir = new File(path)
         if (dir.isDirectory()) {
           dir.listFiles().foreach { file =>
-            if (!hadoopConfFiles.contains(file.getName())) {
+            if (file.isFile && !hadoopConfFiles.contains(file.getName())) {
               hadoopConfFiles(file.getName()) = file
             }
           }