Add Stats collector service for Scaler.

 Currently reports list of nodes with capacity - memory and cpu.

Periodically collects and reports  mean, max, and 90p usage stats for memory and cpu.
This is in a hope that we'll have multiple callers for discovery. We don't need to hit the kube API again if we just refreshed data. Since we refresh per minute, this won't have any effect in case of single caller.