[SPARK-2406][SQL] Initial support for using ParquetTableScan to read HiveMetaStore tables.

This PR adds an experimental flag `spark.sql.hive.convertMetastoreParquet` that when true causes the planner to detects table that use Hive's Parquet SerDe and instead plans them using Spark SQL's native `ParquetTableScan`.
I am going to test this PR soon. In the meantime would it make sense to only put this in `SQLConf` (as well as a field of the key string in the singleton object), making that class the central place that stores SQL configs?