Create database model for subnets and shared networks

We need to support the model in which the database holds a single instance of a given subnet and multiple servers can be associated with it, e.g. in a load balancing case. We should be able to edit this subnet and then deploy it to the selected DHCP servers. As a first step (with this ticket) we should be able to detect the subnets from the Kea servers` configurations and store them in the dedicated tables.