DeploymentStep: now has waitTime and a check function
DeploymentAction: now has tasks to kill
Merge branch 'wip-deployment-features' of https://github.com/mesosphere/marathon into wip-deployment-features

# By Dario Rexin
# Via Dario Rexin
* 'wip-deployment-features' of https://github.com/mesosphere/marathon:
  make EventBus mandatory
  added events and fixed callback for startup