Add option to g-s-s that allows for syncing images only for use by octavia-diskimage-retrofit

[Drew Freiberger]

When discussing the need to deploy g-s-s to support the octavia-diskimage-retrofit charm, a customer raised the request to have g-s-s only sync an image for use by the amphora creation process but not available to the remaining cloud users.

It would be a useful feature to allow configuration of which project images are loaded into and whether they are marked shared/public or private.