'Validate configuration.
Validate that network_device in excluded_device
exists in device mappings'

def _validate(self):

    dev_net_set = set(itertools.chain.from_iterable(six.itervalues(self.device_mappings)))
    for dev_name in self.exclude_devices.keys():
        if (dev_name not in dev_net_set):
            raise ValueError((_('Device    name    %(dev_name)s    is    missing    from    physical_device_mappings') % {'dev_name': dev_name}))
