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
Added docs for `DELETE /v2/deployments{id}?force`
Merge pull request #767 from mesosphere/755-delete-deployment-no-rollback

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