Move application-specific container allocation logic from LeafQueue to FiCaSchedulerApp

Have a discussion with vinodkv and jianhe:

In existing Capacity Scheduler, all allocation logics of and under LeafQueue are located in LeafQueue.java in implementation. To make a cleaner scope of LeafQueue, we'd better move some of them to FiCaSchedulerApp.

Ideal scope of LeafQueue should be: when a LeafQueue receives some resources from ParentQueue (like 15% of cluster resource), and it distributes resources to children apps, and it should be agnostic to internal logic of children apps (like delayed-scheduling, etc.). IAW, LeafQueue shouldn't decide how application allocating container from given resources.