[SPARK-14204][SQL] register driverClass rather than user-specified class

This is a pull request that was originally merged against branch-1.6 as #12000, now being merged into master as well.  srowen zzcclp JoshRosen

This pull request fixes an issue in which cluster-mode executors fail to properly register a JDBC driver when the driver is provided in a jar by the user, but the driver class name is derived from a JDBC URL (rather than specified by the user). The consequence of this is that all JDBC accesses under the described circumstances fail with an IllegalStateException. I reported the issue here: https://issues.apache.org/jira/browse/SPARK-14204

My proposed solution is to have the executors register the JDBC driver class under all circumstances, not only when the driver is specified by the user.

This patch was tested manually. I built an assembly jar, deployed it to a cluster, and confirmed that the problem was fixed.

Author: Kevin McHale <kevin@premise.com>

Closes #14420 from mchalek/mchalek-jdbc_driver_registration.