text: Title: Need a better way to switch backend
  Body: 
  
  Currently, it is not easy to switch to the ray execution backend.
  
  We don't want to introduce lots of the new APIs, such as `new_cluster`, `new_ray_session` in the https://github.com/mars-project/mars/blob/master/mars/deploy/oscar/ray.py for the mars on ray.
  
  Instead, we want to reuse the mars APIs.
  
  For Mars,
  ```python
  new_cluster(worker_num=2, worker_cpu=2)  # The default backend is mars
  ```
  
  For Ray,
  ```python
  new_cluster(backend="ray", worker_num=2, worker_cpu=2)
  ```
  
  
  
  