[SPARK-8020] [SQL] Spark SQL conf in spark-defaults.conf make metadataHive get...

[SPARK-8020] [SQL] Spark SQL conf in spark-defaults.conf make metadataHive get constructed too early

https://issues.apache.org/jira/browse/SPARK-8020

Author: Yin Huai <yhuai@databricks.com>

Closes #6571 from yhuai/SPARK-8020-1 and squashes the following commits:

0398f5b [Yin Huai] First populate the SQLConf and then construct executionHive and metadataHive.