Add a task_id -> container_id mapping in state.json

Currently, there is no way to get the container-id of a task from hitting the mesos master alone. You must first hit the master to get the task_id -> agent_id and task_id -> executor_id mappings, then hit the corresponding agent with agent_id to get the executor_id -> container_id mapping.

It would simplify things alot if the container_id information was immediately available in the /tasks and /state endpoints of the master itself.