Update base.py to group data by collectd types

Hi Ricardo,

I have modified your code to group collectd data types (create data-sets).
As default you use "gauge" type, all metrics was mixed in my graphs (I use CGraphZ to display collectd datas).

I have declared new types in a collectd types.db.ceph file (and include this new types in my collectd.conf).

I don't know the impact of this modification on your grafana dashboard...

You are free to merge this modification (or not ;) )

Cheers,

Sebastien