Augment rds clusters with their respective tags.

def _rds_cluster_tags(model, dbs, session_factory, generator, retry):
    """Augment rds clusters with their respective tags."""
    client = local_session(session_factory).client('rds')

    def process_tags(db):
        try:
            db['Tags'] = retry(
                client.list_tags_for_resource,
                ResourceName=generator(db[model.id]))['TagList']
            return db
        except client.exceptions.DBClusterNotFoundFault:
            return None

    # Rds maintains a low api call limit, so this can take some time :-(
    return list(filter(None, map(process_tags, dbs)))