Refactor postfix::sasl_auth

This addresses issue 7672 (in our internal redmine)

Refactor postfix::sasl_auth:
- Recipe is run automatically if smtp_sasl_passwords is set
- Default smtp_sasl_password_maps value provided so you don't have to
  manually set it

Similarly, add default sender_dependent_relayhost_maps so you don't have to
manually set it