[SPARK-15034][SPARK-15035][SPARK-15036][SQL] Use spark.sql.warehouse.dir as the warehouse location

This PR contains three changes:
1. We will use spark.sql.warehouse.dir set warehouse location. We will not use hive.metastore.warehouse.dir.
2. SessionCatalog needs to set the location to default db. Otherwise, when creating a table in SparkSession without hive support, the default db's path will be an empty string.
3. When we create a database, we need to make the path qualified.

Existing tests and new tests

Author: Yin Huai <yhuai@databricks.com>

Closes #12812 from yhuai/warehouse.