[SPARK-20059][YARN] Use the correct classloader for HBaseCredentialProvider

## What changes were proposed in this pull request?

Currently we use system classloader to find HBase jars, if it is specified by `--jars`, then it will be failed with ClassNotFound issue. So here changing to use child classloader.

Also putting added jars and main jar into classpath of submitted application in yarn cluster mode, otherwise HBase jars specified with `--jars` will never be honored in cluster mode, and fetching tokens in client side will always be failed.

## How was this patch tested?

Unit test and local verification.

Author: jerryshao <sshao@hortonworks.com>

Closes #17388 from jerryshao/SPARK-20059.