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

This is another implementation about #1031
Hmmm... I wonder if this should be done. The way it is, the code creating the `Command` instance can choose the command's environment (e.g., it can set it to `sys.env` to inherit the environment or can create something completely different). Here you're forcing the caller's environment onto the child `Command`...