Adding a cluster usage based scaling policy.

This will serve as an example to add more complex auto scaling policies in the future.
Sorry, the re-scheduler come just points to the race. As in, we don't have a way to affect the load of the cluster by adding machines. We only affect the future tasks that get added.

But as you have it implemented right now I think it'll be okay. It means we will always have 10% capacity with available resources which sounds good :)