puppet: Allow manual configuration of postfix_mailname.

This allows users to configure a mailname for postfix in
/etc/zulip/zulip.conf

puppet/zulip/manifests/postfix_localmail.pp

@@ -4,6 +4,7 @@ class zulip::postfix_localmail {
   if $fqdn == '' {
     fail("Your system does not have a fully-qualified domain name defined. See hostname(1).")
   }
+  $postfix_mailname = zulipconf("postfix", "mailname", $fqdn)
   package { $postfix_packages:
     ensure => "installed",
     require => File['/etc/mailname'],

puppet/zulip/templates/postfix/main.cf.erb

@@ -20,7 +20,7 @@ alias_maps = hash:/etc/aliases
 alias_database = hash:/etc/aliases
 transport_maps = hash:/etc/postfix/transport
 myorigin = /etc/mailname
-mydestination = localhost, <%= @fqdn %>
+mydestination = localhost, <%= @postfix_mailname %>
 relayhost =
 mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
 mailbox_size_limit = 0