fix default value for gitlab-server to include scheme and port

I ran into this while playing with gdk. Pages is defaulting the value of -gitlab-server to be the host part of -artifacts-server.

This means that if -gitlab-server is not set, and -artifacts-server is http://127.0.0.1:3000/api/v4 -- then -gitlab-server will be set to 127.0.0.1.

This is invalid, it actually needs to include the host, and preferrably the port. This patch changes the behaviour so it becomes http://127.0.0.1:3000.