install-server: replace lighttpd with nginx

nginx won the event-based-webservers war and we're already using it
widely in other parts of the infrastructure. Kill lighttpd here.

This doesn't handle the replacement gracefully within puppet and will
fail if applied directly since both servers will try to bind to port 80.
Since the class is only applied in one host (carbon), it's just simpler
to do it manually.

Update the RSpecs to test for the /srv/index.html file and update the
Rakefile to use wmflib and nginx module.

Change-Id: I71ca70321f847ffc6423b827033650407d82c7ef