matrix: Use 127.0.0.1 for smtp_host

Not sure if this is relevant, but postfix only listens on IPv4. This is
what I just tested as working, so keep it this way.

roles/matrix/templates/homeserver.yaml.j2

@@ -1985,7 +1985,7 @@ password_config:
 email:
   # The hostname of the outgoing SMTP server to use. Defaults to 'localhost'.
   #
-  smtp_host: localhost
+  smtp_host: 127.0.0.1
 
   # The port on the mail server for outgoing SMTP. Defaults to 25.
   #