[Metatype] Default value and options

There will be a clarification in the Metatype specification for attribute descriptors (AD): If an AD has a list of option values, the default value should in fact be one of these listed values. Implementations will be required to validate this and discard the default value if not on the list.

Currently our implementation just takes the default value(s). We should change and validate the values and discard those not being part of the option list.