Replace to_dict() calls with a function decorator

Replace to_dict() calls with a function decorator

Up until now, API server functions would need to return simple iterable
objects, such as dicts and lists of dicts. This patch introduces a
decorator which allows such functions to return non-simple objects (as
long as the returned object implements the 'to_dict()' method, or is a
list of such objects) and converts them on its own, simplifying the
user's code and removing code duplication.

Change-Id: Ib30a9213b86b33826291197cf01f00bc1dd3db52