Retrieve expected sentinel value for an output, expanding records.

def _get_sentinel_val(v):
    """Retrieve expected sentinel value for an output, expanding records.
    """
    out = workflow.get_base_id(v["id"])
    if workflow.is_cwl_record(v):
        out += ":%s" % ";".join([x["name"] for x in _get_record_fields(v)])
    return out