config: fix hanging alert about missing name

At the last itaration of review it was decided to throw an alerts,
when no UUID was passed to config and name is not set in _cluster.

This leads to alerts, thrown during replicaset bootstrap. However,
_cluster:on_replace trigger wasn't updated for that change, it
asssumed that when insert of a new replica is done, no alerts was
thrown.

Let's fix the behavior of on_replace trigger so that it deletes
alerts as soon as replica joined with name.

Follow-up #8978

NO_DOC=bugfix
NO_TEST=<already tested>
NO_CHANGELOG=bugfix

src/box/lua/config/applier/box_cfg.lua

@@ -101,7 +101,8 @@ local function names_alert_missing(config, missing_names)
     end
 
     local unknown_msg = 'box_cfg.apply: instance %s is unknown. Possibly ' ..
-                        'instance_name is not set and UUID is not specified'
+                        'instance_name is not set in database and UUID is ' ..
+                        'not specified. Or instance have not joined yet.'
     for name, uuid in pairs(missing_names._peers) do
         local warning
         if uuid == 'unknown' then
@@ -213,22 +214,11 @@ local function names_schema_upgrade_on_replace(old, new)
 end
 
 local function names_cluster_on_replace(old, new)
-    -- Ignore insert of a new replica. Every new replica will have
-    -- name set, as it may join to a replicaset only by bootstrap, which
-    -- means that config will pass instance_name to box.cfg. If user joins
-    -- replica without config module, we consider he understands, what he's
-    -- doing and don't automatically set instance_name.
-    if old == nil then
-        return
-    end
-
-    -- Name may be nil, if e.g. UUID have not been passed to config.
-    -- In such case alert was not done, it's safe to ignore such case.
     local instance_name = nil
-    if new == nil then
-        instance_name = names_state.config._configdata:peer_name_by_uuid(old[2])
-    elseif old[3] == nil then
+    if new ~= nil then
         instance_name = new[3]
+    elseif old ~= nil then
+        instance_name = names_state.config._configdata:peer_name_by_uuid(old[2])
     end
 
     if instance_name ~= nil then