scope script: Allow 'user' part of image name to be given by DOCKERHUB_USER env var
scope script: Allow 'user' part of image name to be given by DOCKERHUB_USER env var

This allows alternate scope images to be used, and mirrors the option available for weave net.

kinvolk/scope,kinvolk/scope,weaveworks/scope,weaveworks/scope,alban/scope,weaveworks/scope,alban/scope,weaveworks/scope,alban/scope,alban/scope,weaveworks/scope,weaveworks/scope,kinvolk/scope,alban/scope,alban/scope,kinvolk/scope,kinvolk/scope,kinvolk/scope