[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.
OK, make sense. 
* Maybe it's better move this line out of function rewritePaths, as it may be called multiple times. 
* In addition, we also have to delete the temporal folder after unit test finished.