puppet: Set /etc/mailname based on postfix.mailname configuration.

The `postfix.mailname` setting in `/etc/zulip.conf` was previously
only used for incoming mail, to identify in Postfix configuration
which messages were "local."

Also set `/etc/mailname`, which is used by Postfix to set how it
identifies to other hosts when sending outgoing email.

Co-authored-by: Alex Vandiver <alexmv@zulip.com>

docs/production/deployment.md

@@ -721,7 +721,8 @@ more than 3.5GiB of RAM, 4 on hosts with less.
 #### `mailname`
 
 The hostname that [Postfix should be configured to receive mail
-at](email-gateway.md#local-delivery-setup).
+at](email-gateway.md#local-delivery-setup), as well as identify itself as for
+outgoing email.
 
 ### `[postgresql]`
 

docs/production/email-gateway.md

@@ -80,9 +80,10 @@ using an [HTTP reverse proxy][reverse-proxy]).
    mailname = emaildomain.example.com
    ```
 
-   This tells postfix to expect to receive emails at addresses ending
-   with `@emaildomain.example.com`, overriding the default of
-   `@hostname.example.com`.
+   This tells postfix to expect to receive emails at addresses ending with
+   `@emaildomain.example.com`, overriding the default of
+   `@hostname.example.com`. It will also identify itself as
+   `emaildomain.example.com` on any outgoing emails it sends.
 
 1. Run `/home/zulip/deployments/current/scripts/zulip-puppet-apply`
    (and answer `y`) to apply your new `/etc/zulip/zulip.conf`

puppet/zulip/manifests/postfix_localmail.pp

@@ -21,7 +21,7 @@ class zulip::postfix_localmail {
     mode    => '0644',
     owner   => root,
     group   => root,
-    content => $fqdn,
+    content => $postfix_mailname,
   }
 
   file {'/etc/postfix/main.cf':