changing volume snapshots to have only one rw image

the current implementation of snapshots allow to write on the base volume image and all the snapshots.
this was implemented because in the past cinder had no way of creating new volumes with the same data of the original volume.
since now we have the ability to create a new volume from an existing volume, an image or a snapshot I think that it's time for this implementation to change and we can start using snapshot as a way of saving the volume data at a specific point in time.