[SPARK-8063] [SPARKR] Spark master URL conflict between MASTER env variable...

[SPARK-8063] [SPARKR] Spark master URL conflict between MASTER env variable and --master command line option.

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

Closes #6605 from sun-rui/SPARK-8063 and squashes the following commits:

51ca48b [Sun Rui] [SPARK-8063][SPARKR] Spark master URL conflict between MASTER env variable and --master command line option.

(cherry picked from commit 708c63bb)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>