[SPARK-8604] [SQL] HadoopFsRelation subclasses should set their output format class

`HadoopFsRelation` subclasses, especially `ParquetRelation2` should set its own output format class, so that the default output committer can be setup correctly when doing appending (where we ignore user defined output committers).

Author: Cheng Lian <lian@databricks.com>

Closes #6998 from liancheng/spark-8604 and squashes the following commits:

9be51d1 [Cheng Lian] Adds more comments
6db1368 [Cheng Lian] HadoopFsRelation subclasses should set their output format class