code: def check_commands_on_units(self, commands, sentry_units):
          
          self.log.debug('Checking exit codes for {} commands on {} '
                         'sentry units...'.format(len(commands),
                                                  len(sentry_units)))
          for sentry_unit in sentry_units:
              for cmd in commands:
                  output, code = sentry_unit.run(cmd)
                  if code == 0:
                      self.log.debug('{} `{}` returned {} '
                                     '(OK)'.format(sentry_unit.info['unit_name'],
                                                   cmd, code))
                  else:
                      return ('{} `{}` returned {} '
                              '{}'.format(sentry_unit.info['unit_name'],
                                          cmd, code, output))
          return None
docs: Check that all commands in a list exit zero on all
          sentry units in a list.
  
          :param commands:  list of bash commands
          :param sentry_units:  list of sentry unit pointers
          :returns: None if successful; Failure message otherwise