Merge branch 'branch-1.1' of github.com:apache/spark into csd-1.1
[branch-1.1][SPARK-4148][PySpark] fix seed distribution and add some tests for rdd.sample

Port #3010 to branch-1.1.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3104 from mengxr/SPARK-4148-1.1 and squashes the following commits:

684c002 [Xiangrui Meng] apply SPARK-4148 to branch-1.1
Merge branch 'branch-1.1' of github.com:apache/spark into csd-1.1