Fix bug in docker-toool where values are sometimes empty.
Fix bug in docker-toool where values are sometimes empty.

I was getting empty volumes from the json and it was creating bad
docker command lines.

Change-Id: Ie90fc1afa5711d6b029e98d621507b9cb70c1dbe

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates