[Reviewer:AMC] Add a CDF line to config template

Just set the CDF to cdf.domain at the moment since we don't have a real cdf

Used as part of sto851 by homestead-prov to specify a ccf when provisioning subscribers

Live tested by spinning up a blank node and checking /etc/clearwater/config
Since we set `ralf` to `""` I think this should also be blank.