[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.
Could this just be:
```scala
val testTempDir = File.createtempfile("", "spark.hive.tmp")
testTempDir.delete()
testTempDir.mkdir()
```

I'm not sure why we would want to keep temporary files in the source tree instead of the system temporary folder.