Networking

Someone should probably read through, @sseago? Let me know if you prefer me to squash first.
Why not just do this:
validates :subnet_type_id, :uniqueness => {:scope => :deployment_id}

This would enforce that, for any given deployment, only one SubnetTyping can exist for a single SubnetType. The subnet ID is not unique, since more than one SubnetType can be attached to a single Subnet. I think the scoped uniqueness validation is equivalent to your custom validation here, unless I've missed something.