[SPARK-12546][SQL] Change default number of open parquet files

A common problem that users encounter with Spark 1.6.0 is that writing to a partitioned parquet table OOMs.  The root cause is that parquet allocates a significant amount of memory that is not accounted for by our own mechanisms.  As a workaround, we can ensure that only a single file is open per task unless the user explicitly asks for more.

Author: Michael Armbrust <michael@databricks.com>

Closes #11308 from marmbrus/parquetWriteOOM.