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 writing it as - 

public class Hosts {
  ImmutableList<String> hosts;

  public Hosts(ImmutableList<String> hosts) {
    this.hosts = hosts;
  }

Do you think it is OK ?