Make NovaObject report changed-ness of its children

This makes NovaObject implementations report the changed-ness of
their child objects. This avoids the case where a child object is
touched and the parent doesn't notice that it needs to be saved
back to the database.

Change-Id: Iabda4206b3662b46dab70c925c661fdedeebc432
Merge "Make NovaObject report changed-ness of its children"