[branch-1.0][SPARK-4148][PySpark] fix seed distribution and add some tests for rdd.sample

Port #3010 to branch-1.0.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3106 from mengxr/SPARK-4148-1.0 and squashes the following commits:

c834cee [Xiangrui Meng] apply SPARK-4148 to branch-1.0