puppet: Only match incoming gateway address on our mail domain.

79931051bd allows outgoing emails from
localhost, but outgoing recipients are still subjected to virtualmaps.
This caused all outgoing email from Zulip with destination addresses
containing `.`, `+`, or starting with `mm`, to be redirected back
through the email gateway.

Bracket the virualmap addresses used for local delivery to the mail
gateway with a restriction on the domain matching the
`postfix.mailname` configuration, regex-escaped, so those only apply
to email destined for that domain.

The hostname is _not_ moved from `mydestination` to
`virtual_alias_domains`, as that would preclude delivery to
actually-local addresses, like `postmaster@`.

puppet/zulip/manifests/postfix_localmail.pp

@@ -45,7 +45,7 @@ class zulip::postfix_localmail {
     mode    => '0644',
     owner   => root,
     group   => root,
-    source  => 'puppet:///modules/zulip/postfix/virtual',
+    content => template('zulip/postfix/virtual.erb'),
     require => Package[postfix],
     notify  => Service['postfix'],
   }

puppet/zulip/templates/postfix/virtual.erb

@@ -1,6 +1,7 @@
+if /@<%= Regexp.escape(@postfix_mailname) %>\.?$/
 # Changes to this list require a corresponding change to `access` as
 # well.
-
 /\+.*@/  zulip@localhost
 /\..*@/  zulip@localhost
 /^mm/    zulip@localhost
+endif