postfix: references non-existent sasl_passwd file

See https://www.howtoforge.com/community/threads/error-in-mail-log-with-some-emails.86297/

smtp_sasl_password_maps specifies hash:/etc/postfix/sasl_passwd (from !1389 (merged), copied from !713 (closed)) - a common filename used from examples, but not one ispconfig creates, hence missing filename messages.