Backport: train
Commit efb8b8bd27d08469c94f103b7314e9905cf6aa0e introduced in Train uses
docker_storage_driver in a Kayobe top-level playbook, but this variable
was only defined in defaults of the docker-devicemapper role. Unless
docker_storage_driver is explicitly set by the operator, the
docker-devicemapper playbook would fail due to the variable being
undefined.
Change-Id: I5e8219d1807b1a4e3ec65f5478eb3f1c7c02b241
Story: 2007719
Task: 39857
4ba18116ff