Python MLOutput.getDF() Can't Access JVM SQLContext

In PySpark, access to the JVM SQLContext from a PySpark SQLContext instance has changed from sqlContext._scala_SQLContext to sqlContext._ssql_ctx has always been officially exposed via sqlContext._ssql_ctx. However, we have been using an unofficial variable, sqlContext._scala_SQLContext, which has been renamed in 2.0, breaking any previous code using the former construct, such as our Python MLOutput.getDF(...) method. Therefore, we just need to update our PySpark API to use the official access point.