Moshe refactor riemann prefix feature

optional fields are checked in the routeros (port is optional):
```C
    if (strcasecmp ("Host", child->key) == 0)
      status = cf_util_get_string (child, &router_data->node);
    else if (strcasecmp ("Port", child->key) == 0)
      status = cf_util_get_service (child, &router_data->service);
    .
    .
    .
    else
    {
      WARNING ("routeros plugin: Unknown config option `%s'.", child->key);
    }
    if (status != 0)
      break;
  }
```