cb_cmds: add support for the server tags in the subnets

We need to extend the cb_cmds to facilitate associating the subnets with the server tags. See the following link for command examples: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#remote-subnet4-del-by-id.

This ticket covers both DHCPv4 and DHCPv6.

Edited Jun 30, 2019 by Marcin Siodelski

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