Networking

Someone should probably read through, @sseago? Let me know if you prefer me to squash first.
I"m not sure we want this. In some cases, (i.e. Neutron without configuring 'external' network) we will have an unused network type. Is there any harm in leaving this validation off, and allowing unassigned network types? Eventually we'll want to add an is_required? flag to SubnetType, and then this validation can only ensure that *required* subnet types are selected.