Add support for sharing images between groups of tenants

Currently, Glance has only a coarse idea of ownership. Images are either public or not public, with public images shown in calls to GET /images and GET /images/details and private images shown only when calling GET /images/<ID>.

We would like to add the ability to have shared image groups. This is dependent on the authentication integration, since we first need an authentication "tenant" in order to relate a set of images to a tenant and allow images to be shared with other tenants.