Strip the trailing router prompt from the output.

def strip_prompt(self, a_string):
        """Strip the trailing router prompt from the output."""
        expect_string = r"^(OK|ERROR|Command not recognized\.)$"
        response_list = a_string.split(self.RESPONSE_RETURN)
        last_line = response_list[-1]
        if re.search(expect_string, last_line):
            return self.RESPONSE_RETURN.join(response_list[:-1])
        else:
            return a_string