Make nova-network use Network to create networks

This makes nova-network manager use the Network.create() method
to do the work of creating networks. It also makes the rpcapi
use the NovaObjectSerializer since this is the first change that
makes nova-network return or take an object via RPC.

Related to blueprint nova-network-objects

Change-Id: Ida9a05ac249ff168075f87e5dbdad6bc23e69561
Merge "Make nova-network use Network to create networks"