DHCP and multiple interfaces

Currently, we only allow users to select one interface on which AGH serves DHCP. That model doesn't quite work when you have e.g. a router that also has some machines connected to it over WAN.

We could either allow multiple DHCP setting sets or maybe add a new pseudointerface value like “Serve on all interfaces”.

Either way, this may complicate some other parts of the logic, for example the selection of the gateway IP address and IP ranges.

@ameshkov, please tell us what you think.