[SPARK-10971][SPARKR] RRunner should allow setting path to Rscript.

Add a new spark conf option "spark.sparkr.r.driver.command" to specify the executable for an R script in client modes.

The existing spark conf option "spark.sparkr.r.command" is used to specify the executable for an R script in cluster modes for both driver and workers. See also [launch R worker script](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/api/r/RRDD.scala#L395).

BTW, [envrionment variable "SPARKR_DRIVER_R"](https://github.com/apache/spark/blob/master/launcher/src/main/java/org/apache/spark/launcher/SparkSubmitCommandBuilder.java#L275) is used to locate R shell on the local host.

For your information, PYSPARK has two environment variables serving simliar purpose:
PYSPARK_PYTHON	      Python binary executable to use for PySpark in both driver and workers (default is `python`).
PYSPARK_DRIVER_PYTHON	Python binary executable to use for PySpark in driver only (default is PYSPARK_PYTHON).
pySpark use the code [here](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/PythonRunner.scala#L41) to determine the python executable for a python script.

Author: Sun Rui <rui.sun@intel.com>

Closes #9179 from sun-rui/SPARK-10971.