Spec Lite: Refactor Glance Scrubber

problem: The scrubber code uses the registry v1 client. This is bad because it doesn't support Keystone v3 auth, which is widely used by default. Additionally, when the Images API v1 is removed, nothing else will be using the registry v1 client.
solution: Refactor the scrubber so that it doesn't use the registry at all. Planning for registry deprecation began in Newton and the deprecation is being officially announced in Queens. This refactoring will allow us to remove the registry on schedule in the S release.
impacts: None
timeline: Q-2 milestone or thereabouts (early December)
link: https://blueprints.launchpad.net/glance/+spec/scrubber-refactor
reviewers: rosmaita, abhishekk, jokke
assignee: wangxiyuan