text: Title: [Reservations] Only waits for reservations
  Body: Reservations can be much cheaper than the on-demand instances, once a reservation is purchased for a future period of time, a user wants `sky launch` to only wait the reservation to be ready and launch the job, not risking to get an on-demand cluster with a much higher price.
  
  It could be done by:
  1. Allowing a new value for the `prioritize_reservations` field in `~/.sky/config.yaml`: `reservation_only`
  2. Allowing the `prioritize_reservations` to be specified in SkyPilot yaml, i.e. the experimental section.