Roll up node stats

As the number of providers and targets grows, the number of stats
that graphite has to sum in order to produce the summary graphs that
we use grows.  Instead of asking graphite to summarize something like
400 metrics (takes about 3 seconds), have nodepool directly produce
the metrics that we are going to use.

Change-Id: I2a7403af2512ace0cbe795f2ec17ebcd9b90dd09
Merge "Roll up node stats"