[SPARK-2191][SQL] Make sure InsertIntoHiveTable doesn't execute more than once.

Author: Michael Armbrust <michael@databricks.com>

Closes #1129 from marmbrus/doubleCreateAs and squashes the following commits:

9c6d9e4 [Michael Armbrust] Fix typo.
5128fe2 [Michael Armbrust] Make sure InsertIntoHiveTable doesn't execute each time you ask for its result.

(cherry picked from commit 777c5958c4088182f9e2daba435ccb413a2f69d7)
Signed-off-by: Reynold Xin <rxin@apache.org>