[SPARK-2700] [SQL] Hidden files (such as .impala_insert_staging) should be filtered out by sqlContext.parquetFile

Author: chutium <teng.qiu@gmail.com>

Closes #1691 from chutium/SPARK-2700 and squashes the following commits:

b76ae8c [chutium] [SPARK-2700] [SQL] fixed styling issue
d75a8bd [chutium] [SPARK-2700] [SQL] Hidden files (such as .impala_insert_staging) should be filtered out by sqlContext.parquetFile