cb_cmds: add support for server tags in the shared networks

Extend cb_cmds to facilitate associating the shared networks being added with the server tags. See the following link for the examples: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#remote-network4-set.

This ticket covers both DHCPv4 and DHCPv6.

Edited Jun 30, 2019 by Marcin Siodelski

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