Check if the pre-built image exists before provisioning the cloud instance

Currently, for me, it's painful to run anything in the cloud as I'm often unsure if the pre-build is done or not.
If i run it without `--build`, it will take long time to provision a GPU instance only to know that the pre-build is not there.
If we are able to check the pre-build before provisioning this will result in a lot better user experience!