Add support for changing the log level at run time through the SparkContext. Based on an earlier PR, #2433 includes CR feedback from pwendel & davies

Author: Holden Karau <holden@pigscanfly.ca>

Closes #5791 from holdenk/SPARK-3444-provide-an-easy-way-to-change-log-level-r2 and squashes the following commits:

3bf3be9 [Holden Karau] fix exception
42ba873 [Holden Karau] fix exception
9117244 [Holden Karau] Only allow valid log levels, throw exception if invalid log level.
338d7bf [Holden Karau] rename setLoggingLevel to setLogLevel
fac14a0 [Holden Karau] Fix style errors
d9d03f3 [Holden Karau] Add support for changing the log level at run time through the SparkContext. Based on an earlier PR, #2433 includes CR feedback from @pwendel & @davies