Make nova-network use Network object for updates

This makes nova-network manager use the Network object (which is
already present in most places) for update operations, in the form
of save().

Related to blueprint nova-network-objects

Change-Id: Icd1a41f1da67d08ecccf8ef9ebc5b9d1e7b28eab
Merge "Make nova-network use Network object for updates"