[SPARK-10711] [SPARKR] Do not assume spark.submit.deployMode is always set

In ```RUtils.sparkRPackagePath()``` we
1. Call ``` sys.props("spark.submit.deployMode")``` which returns null if ```spark.submit.deployMode``` is not suet
2. Call ``` sparkConf.get("spark.submit.deployMode")``` which throws ```NoSuchElementException``` if ```spark.submit.deployMode``` is not set. This patch simply passes a default value ("cluster") for ```spark.submit.deployMode```.

cc rxin

Author: Hossein <hossein@databricks.com>

Closes #8832 from falaki/SPARK-10711.

core/src/main/scala/org/apache/spark/api/r/RUtils.scala

@@ -44,7 +44,7 @@ private[spark] object RUtils {
         (sys.props("spark.master"), sys.props("spark.submit.deployMode"))
       } else {
         val sparkConf = SparkEnv.get.conf
-        (sparkConf.get("spark.master"), sparkConf.get("spark.submit.deployMode"))
+        (sparkConf.get("spark.master"), sparkConf.get("spark.submit.deployMode", "client"))
       }
 
     val isYarnCluster = master != null && master.contains("yarn") && deployMode == "cluster"