Support for centralized NAT

[lightning1]

There need to be major changes to the model and the ui including:

• decentralized VLANs, that are just unique per building
• private subnets on a per-user-basis
• a central public ip pool
• management of network translations
• port forwardings to allow services behind the NAT
• usage of an API provided by the NAT gateway

Additional improvements may include:

• simplifying the host model (e.g. removing interfaces for user hosts)
• do not allow public ips and private subnets to be selected during user creation to suppress race conditions by multiple creations at the same time

[lukasjuhrich]

@lightning1 I see you added a forwarding relation, which reminds me vaguely of what we talked about – Could you perhaps outline what the Idea for the current NAT model is?
Also, it is unclear to me what the technical requirements are – what programs (e.g. the NAT boxes) have to have access to what information (e.g. view on this pg database)?

[ibot3]

@sebschrader made a database schema for this. @lukasjuhrich did you take a look at the new schema yet?