[FIX: SPARK-1376] use --arg instead of --args in SparkSubmit to avoid warning messages

Even if users use `--arg`, `SparkSubmit` still uses `--args` for child args internally, which triggers a warning message that may confuse users:

~~~
--args is deprecated. Use --arg instead.
~~~

@sryza Does it look good to you?

Author: Xiangrui Meng <meng@databricks.com>

Closes #485 from mengxr/submit-arg and squashes the following commits:

5e1b9fe [Xiangrui Meng] update test
cebbeb7 [Xiangrui Meng] use --arg instead of --args in SparkSubmit to avoid warning messages

core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala

@@ -196,7 +196,7 @@ object SparkSubmit {
         childArgs ++= appArgs.childArgs
       } else if (clusterManager == YARN) {
         for (arg <- appArgs.childArgs) {
-          childArgs += ("--args", arg)
+          childArgs += ("--arg", arg)
         }
       }
     }

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -110,7 +110,7 @@ class SparkSubmitSuite extends FunSuite with ShouldMatchers {
     childArgsStr should include ("--executor-memory 5g")
     childArgsStr should include ("--driver-memory 4g")
     childArgsStr should include ("--executor-cores 5")
-    childArgsStr should include ("--args arg1 --args arg2")
+    childArgsStr should include ("--arg arg1 --arg arg2")
     childArgsStr should include ("--queue thequeue")
     childArgsStr should include ("--files file1.txt,file2.txt")
     childArgsStr should include ("--archives archive1.txt,archive2.txt")