Make nova-network use Network object for deleting networks

This makes nova-network manager use the Network object when
deleting networks. Since it results in the same underlying call
to the database, it doesn't affect tests at all.

Related to blueprint nova-network-objects

Change-Id: If45020b8264f05b12657bb27f836b9fc60964cd1
Merge "Make nova-network use Network object for deleting networks"