[Bug] Should try provisioning instance in all availability zones before complaining that no instance available

[ashutoshsaboo]

I was trying to do a sky launch for some model on AWS inf2 instances. When i just pass the --region us-east-1 param without --zone, it just searches in one of the zones and fails when no instance in that first-searched for zone are found. Now when I run the same command with --region us-east-1 --zone us-east-1f it succeeds in spawning the instance. Ideally when I'm not passing --zone it should search in all zones one at a time, and not prematurely fail saying that instance not found.

Version & Commit info:

• sky -v: skypilot, version 1.0.0.dev20240620
• sky -c: skypilot, commit 65bbcf5f45276a8584a76758536cf6c429534240