JIRA: https://issues.apache.org/jira/browse/SPARK-8463

Currently, at the reading path, `DriverRegistry` is used to load needed jdbc driver at executors. However, at the writing path, we also need `DriverRegistry` to load jdbc driver.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6900 from viirya/jdbc_write_driver and squashes the following commits:

16cd04b [Liang-Chi Hsieh] Use DriverRegistry to load jdbc driver at writing path.