Support for outbound SMTP AUTH and TLS

This patch adds some extra parameters (documented in comments) to
configure Postfix to talk to a relayhost that requires SMTP AUTH and/or
TLS.

Also refactored class defaults into `postfix::params` class per style
guide, and parameterized `postfix::config` class in order to get rid of
tons of `scope::lookupvar()` calls in templates.