Since Ganeti version 2.6, Ganeti is aware of private networks. We take
advantage of this by propagating network handling to Ganeti. Network
model is expanded to include information about the type of network
(public routed, physical vlan, private filtered), subnet, gateway, dhcp,
link and mac_prefix.

- Update Network Model and delete NetworkLink model
- Create MacPrefixPool and BridgePool models
- Default constant settings for MacPrefix management