SPARK-1680: use configs for specifying environment variables on YARN

Note that this also documents spark.executorEnv.*  which to me means its public.  If we don't want that please speak up.

Author: Thomas Graves <tgraves@apache.org>

Closes #1512 from tgravescs/SPARK-1680 and squashes the following commits:

11525df [Thomas Graves] more doc changes
553bad0 [Thomas Graves] fix documentation
152bf7c [Thomas Graves] fix docs
5382326 [Thomas Graves] try fix docs
32f86a4 [Thomas Graves] use configs for specifying environment variables on YARN

(cherry picked from commit 41e0a21b22ccd2788dc079790788e505b0d4e37d)
Signed-off-by: Thomas Graves <tgraves@apache.org>