config: don't persist group name as cluster_name

We agreed that it is up to a user how to draw a line between one set of
instances that is considered as a separate cluster and another set of
instances that is assumed as a different cluster. This line is virtual
and there is no sense to impose restrictions until we add some certain
(and consistent) semantic to word 'cluster' in context of tarantool.

Part of #8862

NO_DOC=There is an existing documentation request
       https://github.com/tarantool/doc/issues/3544 and it doesn't
       describe such details. I'll work with the documentation team on
       demand on all the necessary details anyway.