Create an ACL entry for the specified instance.

        :param str instance: The name of the instance to associate the new ACL entry with.
        :param str cidr_mask: The IPv4 CIDR mask for the new ACL entry.
        :param str description: A short description for the new ACL entry.
        :param collector kwargs: (optional) Additional key=value pairs to be supplied to the
            creation payload. **Caution:** fields unrecognized by the API will cause this request
            to fail with a 400 from the API.

def create(self, instance, cidr_mask, description, **kwargs):
        """Create an ACL entry for the specified instance.

        :param str instance: The name of the instance to associate the new ACL entry with.
        :param str cidr_mask: The IPv4 CIDR mask for the new ACL entry.
        :param str description: A short description for the new ACL entry.
        :param collector kwargs: (optional) Additional key=value pairs to be supplied to the
            creation payload. **Caution:** fields unrecognized by the API will cause this request
            to fail with a 400 from the API.
        """
        # Build up request data.
        url = self._url.format(instance=instance)
        request_data = {
            'cidr_mask': cidr_mask,
            'description': description
        }
        request_data.update(kwargs)

        # Call to create an instance.
        response = requests.post(
            url,
            data=json.dumps(request_data),
            **self._default_request_kwargs
        )

        # Log outcome of instance creation request.
        if response.status_code == 200:
            logger.info('Successfully created a new ACL for instance {} with: {}.'
                        .format(instance, request_data))
        else:
            logger.info('Failed to create a new ACL for instance {} with: {}.'
                        .format(instance, request_data))

        data = self._get_response_data(response)
        return self._concrete_acl(data)