Removes active health checks when no longer leader

- Adds HealthCheckManager.removeAll
- Improves HealthCheckManager.removeAllFor
- Normalizes logging in HealthCheckActor
- Fixes #356
Cleanup in MarathonScheduler.reconcileTasks
Relocated health check (de)activation call sites
Merge pull request #357 from mesosphere/issue-356-stop-non-leader-hc

Stop task health checks when no longer leader