[SPARK-14124][SQL] Implement Database-related DDL Commands

#### What changes were proposed in this pull request?
This PR is to implement the following four Database-related DDL commands:
 - `CREATE DATABASE|SCHEMA [IF NOT EXISTS] database_name`
 - `DROP DATABASE [IF EXISTS] database_name [RESTRICT|CASCADE]`
 - `DESCRIBE DATABASE [EXTENDED] db_name`
 - `ALTER (DATABASE|SCHEMA) database_name SET DBPROPERTIES (property_name=property_value, ...)`

Another PR will be submitted to handle the unsupported commands. In the Database-related DDL commands, we will issue an error exception for `ALTER (DATABASE|SCHEMA) database_name SET OWNER [USER|ROLE] user_or_role`.

cc yhuai andrewor14 rxin Could you review the changes? Is it in the right direction? Thanks!

#### How was this patch tested?
Added a few test cases in `command/DDLSuite.scala` for testing DDL command execution in `SQLContext`. Since `HiveContext` also shares the same implementation, the existing test cases in `\hive` also verifies the correctness of these commands.

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

Closes #12009 from gatorsmile/dbDDL.