Extend the subnet data model with DHCP parameters

The #931 (closed) introduces subnet view. However, Stork data model includes a limited number of parameters. We need to extend the database model to include additional parameters. Some of the parameters are trivial, e.g. valid lifetime, but some are more complex (e.g., client classes, pools and options). The data model must make it efficient to query a subnet along with its DHCP parameters. We should also differentiate some of the parameters between different servers (use a local subnet concept, like we do for local host).