[WIP][SPARK-1720][SPARK-1719] use LD_LIBRARY_PATH instead of -Djava.library.path

This is another implementation about #1031
Just an observation that this won't work if the launcher process and target machine are running different OSes. Not sure whether that's supported, and I'm sure this is done in other places, but I've seen reports of people being bit by this (especially those using Windows).