Untie the Nova network models

There are several places in nova where network information has been foreign key tied together or to instance structures. These ties are incompatible with external network services.

Take for example an instance that instance has a backref to fixed_ips. As a result instance['fixed_ips'] can be used all throughout the code base. If those fixed_ips live in an external service this causes issues.

The structures that I see are Instance, virtual_interface, fixed_ip, floating_ip, and network. These tables need to have foreign keys pointing to each other removed and the code that uses these references needs to be updated to either request information through the network api, or to refer to the instance network_info cache (there is another blueprint regarding this cache).