install-server: enable IPv6 on lighttpd

carbon now has a AAAA, enable IPv6 on lighttpd so that that its services
(including apt.wikimedia.org and serving preseed.cfg) work as intended.

Change-Id: Ifc5867d5744455ba9a97393f825b1b2a95468e5e
Merge "install-server: enable IPv6 on lighttpd" into production