config: unify logs and alerts inside creds applier

With this patch the following log/alert structure is introduced:
* all informational messages (e.g. some privs granted) use `log.verbose()`
* all warnings about postponed apply use `config:_alert{type = 'warn'}`
* all apply errors are reported with `config:_alert{type = 'error'}`

Debug logs are removed.

Part of #8967

NO_DOC=logging and alerts
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC