Refactor metadef ORM classes to use to_dict instead of as_dict

The Metadef ORM base dictionary class should replace as_dict with to_dict
to be consistent with other Glance ORM classes.

Change-Id: I1550bd519f100cdffcf5046e9838cf192d4aac50
Closes-Bug: 1370058
Merge "Refactor metadef ORM classes to use to_dict instead of as_dict"