Refactor postfix::sasl_auth and send_dependent_relayhosts

- 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
Merge pull request #161 from nhanb/postfix-password

Refactor postfix::sasl_auth