[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

(cherry picked from commit b7c89a7f0ca73153dce36e0f01b81a3947ee1189)
Signed-off-by: Michael Armbrust <michael@databricks.com>