Additional "Unify Manager._update behaviour" cleanup

Make _update always return an instance of self.resource_class.

This is in preparation for fixing bug 1145768 by reverting the API
changes made in the original "Unify Manager._update behaviour" change.

This is to avoid needing to revert some of the cleanup that was made
in that change.

Change-Id: I842bda40a0dc168689a7dd8b88433c3cebef5839
Revert API changes in "Unify Manager._update behaviour"

This revert some of commit 63073104665ee4597cf3b7aa8dc2295a8a7db794,
specifically the changes made to the API since those changes were
never merged in Nova.

Resolves bug 1145768.

Change-Id: I8f83c5a33cfed0c3a659f5221b8b2e730ca9463f