node "www.example.com" {
        $postfixConfig = "none"
        include postfix
}

class postfix {
    ...
        if ! $postfixConfig {
                $postfixConfig = default
        }
        include("postfix::config::${postfixConfig}")

}

class postfix::config::none {
}

class postfix::config::default {
        file { "/etc/postfix/main.cf":
                ensure  => "present",
                content => template("postfix/default.main.cf.erb"),
                require => Package["postfix"],
                notify  => Service["postfix"],
        }
}