Require gitaly_address instead of path in storage settings

What does this MR do and why?

#375254 (closed)

Requiring path in this context is a bit confusing because gitlab-rails does not actually determine where repositories are stored, gitaly does. Omnibus tends to hide this fact by using the same settings to configure gitaly and to configure gitlab-rails.

So here we begin to untangle this configuration by ensuring that gitlab-rails has the configuration that it needs.

Changelog: changed

How to set up and validate locally

1. Edit config/gitlab.yml and remove all path: in production.repositories (not test.repositories).

     ## Repositories settings
     repositories:
       storages: # You must have at least a `default` storage path.
         default:
           gitaly_address: unix:/home/james/src/gitlab-org/gdk/gitaly.socket
         gitaly-1:
           gitaly_address: unix:/home/james/src/gitlab-org/gdk/gitaly.socket

2. Restart gdk

   $ gdk restart

3. Try and view/clone repositories. Everything should still work.

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