Sends Foreman checks periodically.

def _ForemanOp(self):
    """Sends Foreman checks periodically."""
    period = config.CONFIG["Client.foreman_check_frequency"]
    self._threads["Worker"].SendReply(
        rdf_protodict.DataBlob(),
        session_id=rdfvalue.FlowSessionID(flow_name="Foreman"),
        require_fastpoll=False)
    time.sleep(period)