[SPARK-8019] [SPARKR] Support SparkR spawning worker R processes with a command other then Rscript

This is a simple change to add a new environment variable
"spark.sparkr.r.command" that specifies the command that SparkR will
use when creating an R engine process.  If this is not specified,
"Rscript" will be used by default.

I did not add any documentation, since I couldn't find any place where
environment variables (such as "spark.sparkr.use.daemon") are
documented.

I also did not add a unit test.  The only test that would work
generally would be one starting SparkR with
sparkR.init(sparkEnvir=list(spark.sparkr.r.command="Rscript")), just
using the default value.  I think that this is a low-risk change.

Likely committers: shivaram

Author: Michael Sannella x268 <msannell@tibco.com>

Closes #6557 from msannell/altR and squashes the following commits:

7eac142 [Michael Sannella x268] add spark.sparkr.r.command config parameter

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