Call proxied collection before invoking creator in associationlist.append()

Call proxied collection before invoking creator in associationlist.append()

Improved the association proxy list collection so that premature
autoflush against a newly created association object can be prevented
in the case where ``list.append()`` is being used, and a lazy load
would be invoked when the association proxy accesses the endpoint
collection.  The endpoint collection is now accessed first before
the creator is invoked to produce the association object.

Change-Id: I008a6dbdfe5b1c0dfd02189c3d954d83a65f3fc5
Fixes: #3941