[SPARK-20981][SPARKSUBMIT] Add new configuration spark.jars.repositories as...

[SPARK-20981][SPARKSUBMIT] Add new configuration spark.jars.repositories as equivalence of --repositories

## What changes were proposed in this pull request?

In our use case of launching Spark applications via REST APIs (Livy), there's no way for user to specify command line arguments, all Spark configurations are set through configurations map. For "--repositories" because there's no equivalent Spark configuration, so we cannot specify the custom repository through configuration.

So here propose to add "--repositories" equivalent configuration in Spark.

## How was this patch tested?

New UT added.

Author: jerryshao <sshao@hortonworks.com>

Closes #18201 from jerryshao/SPARK-20981.