[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 708c63b)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Author: Sun Rui

R/pkg/inst/profile/shell.R

@@ -24,7 +24,7 @@
   old <- getOption("defaultPackages")
   options(defaultPackages = c(old, "SparkR"))
 
-  sc <- SparkR::sparkR.init(Sys.getenv("MASTER", unset = ""))
+  sc <- SparkR::sparkR.init()
   assign("sc", sc, envir=.GlobalEnv)
   sqlContext <- SparkR::sparkRSQL.init(sc)
   assign("sqlContext", sqlContext, envir=.GlobalEnv)