Replicate image bits to backend locations

Glance v2 API currently supports multiple locations per image. However the workflow to copy the image bits to secondary locations for a single image requires the operator / admin to manually copy those image bits and then use the glance API to add the new location to the image.

While this process suffices for a number of use cases, it's often desirable to replicate image bits within a single openstack deployment. Motivations for doing so include placing image bits based on locality (ex: per AZ) as well as increasing fault tolerance by having multiple copies of the image bits.

This blueprint intends to cover an approach for supporting image replication within a single glance service. It will seek to support the following high-level use case:

"As an operator, I want glance to support operations which allow me to replicate image bits to secondary glance store locations which I have preconfigured within the glance service."

NB: This BP does not intend to copy replicating image bits cross-region. It's targeting image bits replication to store locations within a single OpenStack deployment.