'Generate another node certificate.
:rtype: CertAndKey
:return: The newly created node certificate.'

def add_node(self, index):

    def run(*arguments):
        check_call((['flocker-ca'] + list(arguments)), cwd=self.directory.path)
    run('create-node-certificate')
    tmp_crt = self.directory.globChildren('????????-????-*.crt')[0]
    tmp_key = FilePath((tmp_crt.path[:(-3)] + 'key'))
    crt = self.directory.child(('node-%d.crt' % (index,)))
    key = self.directory.child(('node-%d.key' % (index,)))
    tmp_crt.moveTo(crt)
    tmp_key.moveTo(key)
    cert = CertAndKey(crt, key)
    self.nodes.append(cert)
    return cert
