Add mounts_points to stats

Add a ```{mount_points, [...]}``` section to the stats. This is a list of file system mount points relevant to Riak (specifically, location of ```platform_data_dir```), for use with disk_stats() to determine Riak disk space usage.

Currently, the ```{disk, [...]}``` section of stats lists all of the mount points on the system and their usage. However, there is no way to programmatically tell which of those Riak (the back-ends, specifically) is mounted on.
This PR adds that missing component, to enable the disk usage calculation needed by https://github.com/basho/riak_cs/issues/612