Enable Trove to optionally restict the Cinder backend used for Trove Volumes

It's likely that a Trove admin might want to restrict the type of Cinder backend used for Trove Volumes (e.g. he may want to only use a highly durable backend for database volumes). Cinder allows us to specify a volume_type to be able to do this. This BP suggests making the volume_type param in cinder an optional, configurable parameter to be read from the Trove Config.