[SPARK-19307][PYSPARK] Make sure user conf is propagated to SparkContext.

The code was failing to propagate the user conf in the case where the
JVM was already initialized, which happens when a user submits a
python script via spark-submit.

Tested with new unit test and by running a python script in a real cluster.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #16682 from vanzin/SPARK-19307.