Http Server: File System DF RESTful API

This adds df information based on statfs api. The API provides the block
usage and node usage. Sample exexcution: http://127.0.0.1:8000/fs/df/

Signed-off-by: Shlomi Livne <shlomi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>