Return the Redis key for the SET of keys under a certain bank, given the bank name.

def _get_bank_keys_redis_key(bank):
    '''
    Return the Redis key for the SET of keys under a certain bank, given the bank name.
    '''
    opts = _get_redis_keys_opts()
    return '{prefix}{separator}{bank}'.format(
        prefix=opts['bank_keys_prefix'],
        separator=opts['separator'],
        bank=bank
    )