Allow rules to declare a list of valid values for an option.
For example, a rule like:
CONF = {'allowed-values': list}
... allowed any value to be passed in the list (including bad ones).
It is now possible to declare:
CONF = {'allowed-values': ['value1', 'value2', 'value3']}
... so that the list passed to the options must contain only values in
`['value1', 'value2', 'value3']`.
f65553c4f7