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
seems that this class is immutable... the list of hosts never changes once constructed. So you might as well construct an ImmutableList here, instead of creating a new ImmuatbleList every time get() is called. Same thing for Racks.