[Reservations] Only waits for reservations

[Michaelvll]

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.