[Reviewer: Graeme] Add seagull

Can you please review this change to add the seagull nodes
I'm a bit worried about this. If we deploy seagull as a CDF with a real HSS in our deployment (or vice versa) this will screw up our DNS records. It may be that there isn't an easy solution or that we can live with that (although I can imagine one day want to set up a deployment with seagull as a CDF and an open IMS core HSS.

Actually, it looks as though we don't currently setup an HSS DNS record so we may accidentally get away with that at the moment.