RRD module

This offloads the RRD logic and calls to a separate module and cleans up the rrd.py by moving the RRD and graph definitions of the global and node RRD to separate modules as well.

As the RRD module supports adding new DS if needed (RRD.ensureSanity() resp. RRD.upgrade()), this is the first step towards adding more data to the RRDs.