The following new syntax variants are introduced for `iproto.advertise`.

* `user@` -- use the given user, a password from the `credentials`
  section and `host:port` from `iproto.listen`
* `user:pass@` -- use the given user and password, use `host:port` from
  `iproto.listen`
* `user@host:port` -- use the given user, host and port, use a password
  from the `credentials` section

It allows to don't repeat the same information in different places of
the config.

The `test_no_advertise_*` test cases are generalized: now the same
boilerplate code is used for these and new cases.

The `test.luatest_helpers.server` util gains ability to parse all the
new `iproto.advertise` variants.

The replicaset and etcd configuration examples are updated to use the
`user@` syntax.

Part of #8810

NO_DOC=the old behavior was not released, the documentation request will
       be registered manually
NO_CHANGELOG=see NO_DOC