IMPALA-798: Distributed execution of CTAS and explain CTAS.

Change-Id: I32004a4b31c54cf5c185169fece143a61213d12d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1850
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins