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
Yes, it is not serving any useful purpose. I just left it for the sake of completion since it is also present in Yarn Container class - http://hadoop.apache.org/docs/r2.3.0/api/org/apache/hadoop/yarn/api/records/Container.html

Should I remove it ?