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

This is another implementation about #1031
I feel like this would be better in either of the following forms:

* If `Command.environment` already did this automatically, but I don't know if you can do that in a case class constructor
* If this method were in the `Command` class itself and the `environment` field were private.