Returns the ith default (aws_key_pair_name, key_pair_path).

def key_pair(i, region):
    """Returns the ith default (aws_key_pair_name, key_pair_path)."""
    if i == 0:
        return ("{}_{}".format(RAY, region),
                os.path.expanduser("~/.ssh/{}_{}.pem".format(RAY, region)))
    return ("{}_{}_{}".format(RAY, i, region),
            os.path.expanduser("~/.ssh/{}_{}_{}.pem".format(RAY, i, region)))