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 think this method is a helper that is only used here? In that case... it does not belong into AllocationSpecification.It is rather confusing here, 'cause it is not clear that adds (allocSpec -> runtimeSpec) to the requestsMap.