Parameters like "slave instance list" are very similar to SlapOS Master
reserved slave_instance_list (and slapos.cookbook:slapconfiguration
slave-instance-list) and using them leads to confusion so instead prepare
to use CDN specific frontend-list as the parameter name which shall be passed.

extra_slave_instance_list is still sent as is, in order to being able to drop
it in future iterations of deployed clusters, the new parameter comes in for
future compatbility, and frontend-list is already used and understood by the
instance-slave-list.cfg.in