[SPARK-2652] [PySpark] donot use KyroSerializer as default serializer

KyroSerializer can not serialize customized class without registered explicitly, use it as default serializer in PySpark will introduce some regression in MLlib.

cc mengxr

Author: Davies Liu <davies@databricks.com>

Closes #2916 from davies/revert and squashes the following commits:

43eb6d3 [Davies Liu] donot use KyroSerializer as default serializer

(cherry picked from commit 809c785bcc33e684a68ea14240a466def864199a)
Signed-off-by: Xiangrui Meng <meng@databricks.com>