text: Title: [UX] Skip `STOPPED` cluster when calling `sky stop`
  Body: <!-- Describe the bug report / feature request here -->
  
  We should filter out those clusters in `STOPPED` status  when calling `sky stop`. e.g. for the example below, `sky stop -a` should only stop 2 clusters `lmf-l4-4-machine-image` and `lmf-gcp-l4-4`.
  
  ```bash
  $ sky status
  Clusters
  NAME                    LAUNCHED     RESOURCES                                                                  STATUS   AUTOSTOP  COMMAND                        
  lmf-l4-4-machine-image  14 mins ago  1x GCP(g2-standard-48, {'L4': 4}, image_id={'us-east4': 'projects/skyp...  UP       -         sky launch -c lmf-l4-4-ma...   
  lmf-gcp-l4-4            31 mins ago  1x GCP(g2-standard-48, {'L4': 4})                                          UP       -         sky launch -c lmf-gcp-l4-4...  
  lmf-machine-image       2 hrs ago    1x GCP(g2-standard-24, {'L4': 2}, image_id={'us-east4': 'projects/skyp...  STOPPED  -         sky launch -c lmf-machine...   
  lmf-1-epoch             4 hrs ago    1x GCP(g2-standard-24, {'L4': 2})                                          STOPPED  -         sky launch -c lmf-1-epoch...   
  lmf                     18 hrs ago   1x GCP(g2-standard-24, {'L4': 2})                                          STOPPED  -         sky start lmf                  
  sky-344a-txia           1 month ago  1x Azure(Standard_NV18ads_A10_v5, {'A10': 0.5})                            STOPPED  -         sky exec sky-344a-txia sl...   
  
  Managed jobs
  No in-progress managed jobs. (See: sky jobs -h)
  
  Services
  No live services. (See: sky serve -h)
  $ sky stop -a                     
  Stopping 6 clusters: lmf-l4-4-machine-image, lmf-gcp-l4-4, lmf-machine-image, lmf-1-epoch, lmf, sky-344a-txia. Proceed? [Y/n]:
  ```
  