Better handle cases of cb_cmds with control channel disabled

This config is supposed to showcase CB and CB commands, yet the control channel is disabled, so there's no way to send commands.

Couple things to consider:

• change the config to use control channel (done in #804 (closed)]
• cb_cmds (and all other command providing hooks) should warn if the control channel is disabled (only a warning, though, cb_cmds could be run on a different kea instance)
• consider that perhaps Kea should open the control channel by default and have an ability to disable it somehow