Placement Policy API

1. Added PlacementPolicy API. Only supporting DISTRIBUTED runnables for now.
2. Allocating Containers for one runnable instance at a time (for DISTRIBUTED runnables), instead of grouping them together based on Resource Specs. 
3. Added blacklist support for DISTRIBUTED PlacementPolicy. 
4. Moved Hosts and Racks from ResourceSpecs to PlacementPolicy.
5. Unit tests: Added PlacementPolicyTestRun Class.

Gourav
I think you don't need the `<String>` here.