[SPARK-2652] [PySpark] Turning some default configs for PySpark

Add several default configs for PySpark, related to serialization in JVM.

spark.serializer = org.apache.spark.serializer.KryoSerializer
spark.serializer.objectStreamReset = 100

This will help to reduce the memory usage during RDD.partitionBy()
I've now merged https://github.com/apache/spark/pull/1051, so update this to do _conf.setIfMissing().