None metadata is not allowed

This fixes recent launch failures with error `Launching server failed: TypeError("'NoneType' object does not support item assignment",)`
Summary from IRL/IRC conversation:  nova doesn't seem to have a full schema for the request:  https://github.com/openstack/nova/blob/master/nova/api/openstack/compute/servers.py#L779, so probably not worth importing all of nova just for that metadata schema.  :|