Scheduler Enhancements to support Share Replication in Manila

The Share Replication feature proposed in Mitaka is designed to work with symmetric backends. To support this capability, Manila would need to record, from each backend, the possible "replication_partners" that it can replicate to/from. The Manila scheduler needs to be enhanced to account for this new capability being reported and consider it during the process of finding a suitable host to schedule the replica share's creation.