Spark on Kubernetes tooling hardcodes default image names

Both when creating images with bin/docker-image-tool.sh and when running the Kubernetes integration tests the image names are hardcoded to spark, spark-py and spark-r.

If you are producing custom images in some other way (e.g. a CI/CD process that doesn't use the script) or are required to use a different naming convention due to company policy e.g. prefixing with vendor name (e.g. apache-spark) then you can't directly create/test your images with the desired names.

You can of course simply re-tag the images with the desired names but this might not be possible in some CI/CD pipelines especially if naming conventions are being enforced at the registry level.

It would be nice if the default image names were customisable