Merge pull request #478 from sryza/sandy-spark-1033

SPARK-1033. Ask for cores in Yarn container requests

Tested on a pseudo-distributed cluster against the Fair Scheduler and observed a worker taking more than a single core.
(cherry picked from commit 576c4a4c502ccca5fcd6b3552dd93cc2f3c50666)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>