Add support for mounts on underscored paths

We ran into the problem where trying to grab the free space from the drive mounted at /hadoop/ebs_f failed.  This patch keeps track of the mount points on a system, as well as the aliases that Ganglia assigns to them for display.  It removes the erroneous path translation to /hadoop/ebs/f, and should provide better security against other problematic delimiters in the future.