Refactor CIDR field to use netaddr.IPNetwork

This makes the CIDR field become the IPNetwork field (and associate
versioned fields). It also uses the netaddr.IPNetwork class, which
is what should have been done in the first place.

Wire format is unchanged, so we can make this without any version
bumps or upgrade impact.

Related to blueprint nova-network-objects

Change-Id: I1a3ab6a5f4c624f3da0ab1c78c75c6f1e474541f
Merge "Refactor CIDR field to use netaddr.IPNetwork"