Handle multiple X-Forwarded-For addresses in runner IP

As raised in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24624#note_144220505, the X-Forwarded-For header may contain multiple IPs. Each chain in the proxy will add its own IP.

The naive solution would be just to pick the first IP, but https://github.com/rack/rack/issues/1049#issuecomment-210556173 has an argument that you can't always trust the IPs in the list.

In Rails, this is the algorithm:

https://github.com/rails/rails/blob/a796de1bd644b54e2c27577319c262af0de56e8f/actionpack/lib/action_dispatch/middleware/remote_ip.rb#L112-L150

We couldn't use request.remote_ip because we're coming from Grape, but perhaps we can adapt this method (or contribute it to Grape).