[Core] Launching on a just launched existing cluster with --fast does not skip the provision

[Michaelvll]

To reproduce:

1. sky launch -c test-fast --cloud aws --cpus 2 echo hi
2. sky launch -c test-fast --fast echo hi: does not skip provisioning
3. sky launch -c test-fast --fast echo hi: skips the provisioning

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN

[cg505]

provider.availability_zone in the cluster yaml changes from initial launch (lists all zones) to second and all future launches (only lists the zone that the cluster is actually in)

I will add a mechanism to ignore some fields from the cluster yaml for the config hash calculation. Was trying to avoid that but I think its the best way to go.

[concretevitamin]

Maybe some of these are relevant? https://github.com/skypilot-org/skypilot/blob/master/sky/backends/backend_utils.py#L129-L175