Stale RRD files break the network stats

[cstavr]

The grapher of snf-stats-app computes the average network traffic for all
interfaces of the VM in order to produce the network graph. Collectd generates
a separate RRD file for each network interface, and then snf-stats-app is
using these RRD files in order to calculate the average traffic.

The problem is that when an interface gets removed the corresponding RRD file
is never removed. These stale RRD files seem to cause problem in the
calculation of the average network traffic.

This easy can easily be observed by watching the weekly network graph while
adding and removing a network interface.