[SPARK-11555] spark on yarn spark-class --num-workers doesn't work

I tested the various options with both spark-submit and spark-class of specifying number of executors in both client and cluster mode where it applied.

--num-workers, --num-executors, spark.executor.instances, SPARK_EXECUTOR_INSTANCES, default nothing supplied

Author: Thomas Graves <tgraves@staydecay.corp.gq1.yahoo.com>

Closes #9523 from tgravescs/SPARK-11555.