Add empty volume item check in kolla_docker module

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

ansible/library/kolla_docker.py

@@ -496,6 +496,9 @@ class DockerWorker(object):
         vol_dict = dict()
 
         for vol in volumes:
+            if len(vol) == 0:
+                continue
+
             if ':' not in vol:
                 vol_list.append(vol)
                 continue