falsely log "auth: unknown user" when multiple userdb

[dovecot at ptld.com]

> When there is a multiple userdb to look for user, Dovecot log «auth: unknown user» when the first userdb search failed. 
> Then Dovecot  successfully search user in the second userdb and things works. 
> But  "auth: unknown user" event left in a logs.


On systemd using rsyslog i added this to rsyslog config:

   if $programname == "dovecot" and $msg contains "- trying the next userdb" then stop