config: don't try to persist anon replica name

The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of #9432

NO_DOC=replication.anon option is already documented in the scope of
       https://github.com/tarantool/doc/issues/3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.