We need to carefully set the ouputPartitioning of the HashOuterJoin Operator. Otherwise, we may not correctly handle nulls.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1721 from yhuai/SPARK-2212-BugFix and squashes the following commits:

ed5eef7 [Yin Huai] Correctly choosing outputPartitioning for the HashOuterJoin operator.