The issue is that we need to know what images you have available that match Centos7. I believe you can use “juju metadata generate-image” to create image stream information. Canonical itself doesn’t host Redhat images, which is why we don’t default to having them available.
@wallyworld might have more information on how to set up image metadata. I know we did some work to avoid having to host simplestreams files just to get Juju to work, but I’m not sure how those commands ended up exported (I think they are under a feature flag.)