create a volume

def create(self, volume_id, vtype, size, affinity):
        """
        create a volume
        """
        volume_id = volume_id or str(uuid.uuid4())
        params = {'volume_type_name': vtype,
                  'size': size,
                  'affinity': affinity}
        return self.http_put('/volumes/%s' % volume_id,
                             params=self.unused(params))