Add sasl_password_file attr for 3.1.x postfix cookbook

Since opscode-cookbooks/postfix@14fc7d79191c95538bfc2208ceaea028e7348397 the postfix
cookbook adds a new postfix.sasl_password_file attribute that is only set if 
smtp_sasl_auth_enable is set to yes at the time the postfix cookbook attributes are
parsed - so the definition has to be copied into our attributes.