Introduce qualified UUIDs to support multiple endpoints for a particular service type

OpenStack in general, and Nova specifically, makes the assumption that there is only one endpoint for each type of service. This blueprint outlines some use cases on why supporting multiple endpoints is a desirable feature and proposes some solutions. Fundamentally, the problem is that OpenStack refers to objects (e.g., VMs, volumes and networks) using UUIDs. When there is only a single nova, cinder or neutron, this is fine since each object type is served from only a single endpoint. When there are multiple instances of each of these services with multiple endpoints, however, it is undefined which endpoint owns a particular UUID.