[SPARK-18750][YARN] Avoid using "mapValues" when allocating containers.

That method is prone to stack overflows when the input map is really
large; instead, use plain "map". Also includes a unit test that was
tested and caused stack overflows without the fix.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #16667 from vanzin/SPARK-18750.

(cherry picked from commit 76db394f)
Signed-off-by: Tom Graves <tgraves@yahoo-inc.com>