Register the breakpoint of the name. Usage:
        > breakpoint('setup-done')

        The name should be used in a call of Driver.register_breakpoint(name)
        method, which returns the Deferred, which will be fired by this
        command.

def breakpoint(self, name):
        """
        Register the breakpoint of the name. Usage:
        > breakpoint('setup-done')

        The name should be used in a call of Driver.register_breakpoint(name)
        method, which returns the Deferred, which will be fired by this
        command.
        """
        if name not in self._breakpoints:
            self.warning("Reached breakpoint %s but found no "
                         "callback registered")
            return
        cb = self._breakpoints[name]
        cb.callback(None)
        return cb