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
Simpler:
if (map.containsKey(allocSpec)) { 
  map.get.allocSpec().add(runtimeSpec)) 
} else { 
  map.put(allocSpec, Lists.newLinkedList(runtimeSpec)) 
}

Not sure if you need a hashset or is any collection good enough?