When a user have multiple instances that need to set multiple k8s. Currently, they have to run sky local up multiple times to setup k8s on each of them. We should allow specifying multiple ip list and keys to setup k8s on all of them, e.g.:

clusters:
  my-cluster-1:
    ips:
      - xxx
      - xxx
      - xxx
    key_file: xxx
  my-cluster-2:
    ips:
      - yy
      - yyy
      - yyyy
    key_file: yyy