Allow Specifying the Resource Tracker Class

There has been a lot of discussion recently about ideas to improve the performance and functioning of the scheduler, and one thing that is clear is that there are people for whom the current scheduler is a performance bottleneck, while there are also many who are having no problems and don't want to change how it works. These alternatives usually involve writing the state of compute resources to some alternative storage, which the scheduler could then access to make its decisions. An OpenStack deployment can specify alternate scheduler and host manager classes, but cannot specify a different ResourceTracker class. I propose making this configurable as the other two classes are, by adding the config option 'compute_resource_tracker' to the compute_opts section of nova/compute/manager.py. This option would default to 'nova.compute.resource_tracker.ResourceTracker', so that there will be no change in the default behavior.