Weigh hosts by their number of instances

Since weighing hosts is difficult when you have heterogenous hardware (since it's difficult to find common metrics you can use as comparison), we propose here a simple NumInstancesWeigher that will compare the hosts based on the reported number of instances.

The change is very minimal since the value of instances is already existing in the scheduler HostState, as we already have a NumInstancesFilter that cuts hosts above a threshold.