Allow authenticated SMTP relaying

Hi,

The Landscape charm sets up postfix and allows overriding or specifying a "smarthost" to relay mail (smtp-relay-host)[1]. Can you also allow specifying SMTP AUTH credentials to use for authenticated SMTP relaying?

Something like smtp-relay-user and smtp-relay-password where it would ship out the following configs required to make Postfix use it.

* /etc/postfix/main.cf:

| smtp_sasl_auth_enable=yes
| smtp_sasl_mechanism_filter = plain,login
| smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
| smtp_sasl_security_options = noanonymous
| smtp_tls_security_level = may

* /etc/postfix/sasl_passwd:

| [smtp-server.internal]:25 myserviceuser:anditspassword

(replace smtp-server.internal with what's provided in smtp-relay-host; file should be 600)

Or even in the Landscape On-Prem (aka LDS) application itself, see LP:1969686.

Thanks,

Haw

[1]https://bazaar.launchpad.net/~landscape/landscape-charm/trunk/view/head:/lib/callbacks/smtp.py