Added DELETE /v2/deployments/{id}?force=true

Added DELETE /v2/deployments/{id}?force=true to cancel a deployment without creating a new one
that rolls back to the previous state.

- Adds MarathonSchedulerService.cancelDeployment
- Fixes #755
How about

```scala
case (plan, _) if force =>
  // do not create a new deployment to return to the previous state
  log.info(s"Canceling deployment [$id]")
  service.cancelDeployment(id)
  status(ACCEPTED) // 202: Accepted

case (plan, _) =>
  // create a new deployment to return to the previous state
  deploymentResult(result(groupManager.update(
    plan.original.id,
    _ => plan.original,
    force = true
  )))
```