Add more dictionary methods to VersionedObjectDictCompat:
* __iter__()
* keys(), iterkeys()
* values(), itervalues()
Thanks to new methods, dict constructor now accepts
VersionedObjectDictCompat: it's possible to write directly dict(obj)
instead of having to write dict(obj.iteritems()).
On Python 3, items() now returns an iterator instead of a list, to
respect Python 3 dictionary API, as the new keys() and values() methods.
Keep iterkeys(), itervalues() and iteritems() on Python 3, even if
Python 3 dictionaries don't have these methods, just to ease the
transition from Python 2 to Python 3 in Nova.
Change-Id: I4c52d44c7ba49f98b1bbd123209fce7b70ade98d
3ae09aebff