Backup snapshots

Today we can backup a volume, but not a snapshot.
This patch adds support to backup snapshots and
provide another layer of data protection for the
user.

DocImpact
implements blueprint backup-snapshots

Change-Id: Ib4ab9ca9dc72b30151154f3f96037f9ce3c9c540

cinder/volume/utils.py

@@ -108,6 +108,7 @@ def _usage_from_backup(backup_ref, **kw):
                       fail_reason=backup_ref['fail_reason'],
                       parent_id=backup_ref['parent_id'],
                       num_dependent_backups=num_dependent_backups,
+                      snapshot_id=backup_ref['snapshot_id'],
                       )
 
     usage_info.update(kw)