Hierarchical projects image support - allow image uploads to be visible to child projects

glance currently supports setting an image’s visibility to be public i.e. the image then becomes accessible to all projects. Since we now have the ability to define and use hierarchical projects in keystone, glance needs to allow an image uploaded to a parent project to be shared with all its descendant projects i.e. visibility to be ‘public’ only within its project tree. The current workaround is to mark the image private and explicitly create member access for every child project, but that is not very consumable.
spec in progress