[SPARK-3392] [SQL] Show value spark.sql.shuffle.partitions for mapred.reduce.tasks

This is a tiny fix for getting the value of "mapred.reduce.tasks", which make more sense for the hive user.
As well as the command "set -v", which should output verbose information for all of the key/values.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #2261 from chenghao-intel/set_mapreduce_tasks and squashes the following commits:

653858a [Cheng Hao] show value spark.sql.shuffle.partitions for mapred.reduce.tasks