We move everything to the models. We feature-flag it, because we
probably want to change the names etc, and we aren't going to be able to
offer smooth upgrades until that is done.
* Zones are now subnets
* Utility subnet is no longer part of Zone
* Bastion InstanceGroup type added instead
* Etcd clusters defined in terms of InstanceGroups, not zones
* AdminAccess split into SSHAccess & APIAccess
* Dropped unused Multizone flag