Fix assignment of tasks to dead trackers.

Sometimes after we kill a tracker, we try to assign tasks to it because
the heartbeat expiry period hasn't been reached.
Merge pull request #36 from airbnb/master

Fix assignment of tasks to dead trackers.
Added support for strict role validation

Sometimes the user might want to restrict Hadoop to only launching tasks on top
of resources for the given role, and not accept offers for "*"
Added extra logging
Merge pull request #37 from duedil-ltd/feature/strict-roles

Added support for strict role validation
Initial implementation of idle task tracker monitoring