Implement snapshot instances

We have both snapshot id and snapshot instance id in Manila. The snapshot named by snapshot instance id when user create a snapshot, so user can not quickly find snapshot(on array) according to snapshot id. So add snapshot_instance_list API to show the relationship between snapshot id and snapshot instance id.
API methods will be created:
snapshot_instance_list [snapshot_id]
snapshot_instance_show <snapshot_id>
snapshot_instance_reset_status