[SPARK-15681][CORE] allow lowercase or mixed case log level string when calling sc.setLogLevel

## What changes were proposed in this pull request?
Currently `SparkContext API setLogLevel(level: String) `can not handle lower case or mixed case input string. But `org.apache.log4j.Level.toLevel` can take lowercase or mixed case.

This PR is to allow case-insensitive user input for the log level.

## How was this patch tested?
A unit testcase is added.

Author: Xin Wu <xinwu@us.ibm.com>

Closes #13422 from xwu0226/reset_loglevel.