A empty docker volume item will cause start_container to fail.
This bug is not triggered in current kolla deployment scripts, but
only if you pass a empty volume item to kolla_docker module.
E.g. Using a if expression in items of volumes list while calling
start_container. Like the one in start.yml of iscsi and cinder.
Change-Id: I389246fb9650ab5304463e943459ecb68706167f