Implement a basic backup-volume-to-swift service

Implements: blueprint volume-backups

This patch adds the new service, api and basic unit tests

Change-Id: Ibe02c680c5e9201d208c92e796e86ad76b4b54b3

cinder/exception.py

@@ -537,3 +537,11 @@ class GlanceMetadataExists(Invalid):
 
 class ImageCopyFailure(Invalid):
     message = _("Failed to copy image to volume")
+
+
+class BackupNotFound(NotFound):
+    message = _("Backup %(backup_id)s could not be found.")
+
+
+class InvalidBackup(Invalid):
+    message = _("Invalid backup: %(reason)s")