settings: skip unconfigured values
    
    When a value is not configured in settings map, it is skipped.
    If it is the last processed value, the result of ISC_R_NOTFOUND
    would cause the cleanup section of the function fail. Since
    unconfigured values are allowed and expected, override the result
    to ISC_R_SUCCESS in this case to prevent triggerring an error.
    
    https://pagure.io/bind-dyndb-ldap/issue/172
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>