'Assert that the given fragment of a log line does not appear in the
``/var/log/messages`` file on the control node of the cluster.'

def _assert_not_logged(self, cluster, fragment):

    getting = cluster.get_file(cluster.control_node, FilePath('/var/log/messages'))
    def check_missing_messages(reason):
        reason.trap(RemoteFileNotFound)
        self.skipTest('{}    not    found'.format(reason.value))
    getting.addErrback(check_missing_messages)
    def got_messages(path):
        with path.open() as messages:
            for line in messages:
                self.assertNotIn(fragment, line)
    getting.addCallback(got_messages)
    return getting
