[SPARK-6470] [YARN] Add support for YARN node labels.

This is difficult to write a test for because it relies on the latest version of YARN, but I verified manually that the patch does pass along the label expression on this version and containers are successfully launched.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #5242 from sryza/sandy-spark-6470 and squashes the following commits:

6af87b9 [Sandy Ryza] Change info to warning
6e22d99 [Sandy Ryza] [YARN] SPARK-6470.  Add support for YARN node labels.