[SPARK-7684] [SQL] Invoking HiveContext.newTemporaryConfiguration() shouldn't...

[SPARK-7684] [SQL] Invoking HiveContext.newTemporaryConfiguration() shouldn't create new metastore directory

The "Database does not exist" error reported in SPARK-7684 was caused by `HiveContext.newTemporaryConfiguration()`, which always creates a new temporary metastore directory and returns a metastore configuration pointing that directory. This makes `TestHive.reset()` always replaces old temporary metastore with an empty new one.

Author: Cheng Lian <lian@databricks.com>

Closes #6359 from liancheng/spark-7684 and squashes the following commits:

95d2eb8 [Cheng Lian] Addresses @marmbrust's comment
042769d [Cheng Lian] Don't create new temp directory in HiveContext.newTemporaryConfiguration()