gitlab-ce, omnibus, gdk: move gitaly address under storage shards

We now have a global gitaly.socket_path option in gitlab-ce. This should become an 'address' field under the storage shards.

Current situation in gitlab.yml:

  repositories:
    storages:
      foobar:
         path: /home/git/repositories

  gitaly:
    socket_path: tmp/sockets/private/gitaly.socket

New situation in gitlab.yml:

  repositories:
    storages:
      foobar:
         path: /home/git/repositories
         gitaly_address: unix:/home/git/gitlab/tmp/sockets/private/gitaly.socket

  gitaly:
    enabled: true

This changes settings in gitlab.yml so it will create follow-up work in omnibus and GDK.