Placement Policy API

1. Added PlacementPolicy API. Only supporting DISTRIBUTED runnables for now.
2. Allocating Containers for one runnable instance at a time, instead of grouping them together based on Resource Specs. 
3. Added blacklist support for DISTRIBUTED PlacementPolicy. 
4. TODO: Implement PlacementHints (added placeholder). 
5. Added PlacementPolicyTestRun Class with a method testDistributedPlacementPolicy to test the current implementation.

Gourav
I'm using the constructor in the AM de-serialization method. Hosts and Racks are sent as Set<String> inside json format of TwillSpecs. AM reconstructs Hosts and Racks using the constructor from those Set<String>.