'Executes "signal" command and returns response as a string.
@type signal: str
@param signal: Signal name (SIGHUP, SIGTERM, SIGUSR1 or SIGUSR2)
@rtype: str
@return: Response string'

def signal(self, signal):

    assert (signal in ['SIGHUP', 'SIGTERM', 'SIGUSR1', 'SIGUSR2'])
    return self.execute('signal', signal)[0]
