Current k8s backend requires 3 separate container images to be provided by the user to submit an app. The 3 images are pretty much the same except for the entry point.
Instead, we should require a single image by default, but still allow the user to override individual images (e.g. executor) if desired.
This should also make it easier to publish an "official" Spark docker image, and should simplify things like the build-push-docker-images.sh script.