Instance objects cannot be easily turned into dictionaries

Fixes OS-235

The new nova instance objects cannot be easily converted into
dictionaries by passing them into the dict(...) constructor. This
uses their compatible iteritems() method to extract the key,value
pairs.

Change-Id: Ie4bf93e96473af8c2419d134acea666ba97bd84b
Merge "Instance objects cannot be easily turned into dictionaries" into havana/1.4