Backup: Enable multipathing by default

The default for cinder volume service is to enable multipathing, but we
are missing this on the backup service.

This patch enables multipathing by default for backup services as well.

templates/cinder/config/cinder.conf

@@ -26,6 +26,8 @@ api_paste_config = /etc/cinder/api-paste.ini
 # that would make cinder-api logs be treated as errors by httpd
 log_file = /dev/stdout
 
+use_multipath_for_image_xfer = true
+
 [backend_defaults]
 use_multipath_for_image_xfer = true