[SPARK-2817] [SQL] add "show create table" support

In spark sql component, the "show create table" syntax had been disabled.
We thought it is a useful funciton to describe a hive table.
All we need is the temporal folder, not sure why we have to check the user working directory if end with `sql/hive`, 
Can we just use `val testTmpDir = new File(System.getProperty("user.dir") + File.separator + "tmp")` instead?