Configure Geo node name in gdk.yml

Problem

I ran gdk update because it's part of make geo-primary-update. I think at some point this was safe, and didn't clobber gitlab/config/gitlab.yml, but now it does.

So I had to set

  gitlab:
    geo:
      node_name: gdk-primary

again in my gitlab/config/gitlab.yml. However this is a setup problem for everyone, so I have now opened this MR.

• Allow setting Geo node name in gdk.yml
• Don't clobber gitlab/config/gitlab.yml willy-nilly
• Copy some more gitlab.yml.example settings into gitlab.yml.erb