Host reservations migration from Kea config to the host database

Stork supports managing host reservations in the host database. It doesn't support managing host reservations specified in the Kea config file. Some users may wish to migrate their host reservations from the config file to a database for scalability reasons and to be able to manage them from Stork. This ticket proposes to add migration capability in Stork so the users can get their host reservations from the config file, move them to the Kea host database and remove from the config file.