'returns (action, option, value) candidates for an option prefix
Returns [first candidate] if all candidates refers to current and
deprecated forms of the same options: "nova boot ... --key KEY"
parsing succeed because --key could only match --key-name,
--key_name which are current/deprecated forms of the same option.'

def _get_option_tuples(self, option_string):

    option_tuples = super(NovaClientArgumentParser, self)._get_option_tuples(option_string)
    if (len(option_tuples) > 1):
        normalizeds = [option.replace('_', '-') for (action, option, value) in option_tuples]
        if (len(set(normalizeds)) == 1):
            return option_tuples[:1]
    return option_tuples
