Create the stack user on the machine.

def create_stack_user(self):
        """Create the stack user on the machine.
        """
        self.run('adduser -m stack', success_status=(0, 9))
        self.create_file('/etc/sudoers.d/stack', 'stack ALL=(root) NOPASSWD:ALL\n')
        self.run('mkdir -p /home/stack/.ssh')
        self.run('cp /root/.ssh/authorized_keys /home/stack/.ssh/authorized_keys')
        self.run('chown -R stack:stack /home/stack/.ssh')
        self.run('chmod 700 /home/stack/.ssh')
        self.run('chmod 600 /home/stack/.ssh/authorized_keys')
        self.ssh_pool.build_ssh_client(self.hostname, 'stack',
                                       self._key_filename,
                                       self.via_ip)