Allowing DbUser authentication to pass through to driver

[Gauravshah]

Cloning PR https://github.com/databricks/spark-redshift/pull/374

thanks to @brosander for the code changes

Fixes #62

[lucagiovagnoli]

@Gauravshah thanks a lot for contributing!

Would you mind fixing conflicts and writing at least 1 small test verifying this change works ?

[Gauravshah]

@lucagiovagnoli Sure makes sense. Will redo the PR