node_filesystem variable outdate

[nghyjohn]

New variables should be:

node_filesystem_size_bytes
node_filesystem_free_bytes

[Zauxst]

The node exporter used by the plugin is 0.15.0.
In Node Exporter version 0.16.0 there were a lot of revamps to the node_exporter metrics.

I must add that I too for one would like for the graphs to be updated to the new metrics so we can unify the dashboards that are already configured to use the new variables.

Right now I have to maintain 2 different set of dashboards. to verify the same information.