Allow TaskScheduler manage number slots on TaskTrackers

Currently the map slots and reduce slots are managed by TaskTracker configuration.
To change the task tracker slots, we need to restart the TaskTrackers.
Also, for a non-uniform cluster, we have to deploy different sets of configuration.

Now JobTracker holds the CPU and memory status of TaskTrackers (MAPREDUCE-1218).
So it makes sense to just let JobTracker.taskScheduler decided the number of slots on each node.
This way we can
1. Change the number of slots dynamically without restarting TaskTracker
2. Use different number of slots based on the resource of a TaskTracker

To achieve this, we need to change the logic that we use totalMapSlots and totalReduceSlots in JobTracker.
I think they are used in WebUI and speculativeCap.

We will need to make JobTracker calculate these numbers from TaskScheduler and TaskTrackerStatus.
TaskScheduler and TaskTracker can both hold their maximum slots. We pick the smaller one.

Thoughts?