(9.1) new gitaly config in gitlab.yml

Changes in gitlab.yml introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10150/diffs#0ef14ca7e447f477230136ca76fb4ab0bed43af4

In the repositories.storages section , each entry must get a gitaly_address. By default this will point to the same address for all shards. It must be possible for administrators to override gitaly_address for any storage.

Note that previously we had gitaly.socket_path which was just a path /var/opt/gitlab/bla. Now we must also specify a protocol: unix:/var/opt/gitlab/bla.

  repositories:
    storages: # You must have at least a `default` storage path.
      default:
        path: /home/git/repositories/
        gitaly_address: unix:/home/git/gitlab/tmp/sockets/private/gitaly.socket

We also have a new option gitaly.enabled, which should default to true. An administrator should be able to control the value of this setting (unlike what happened in #2094 (closed)).

Closes #2094 (closed)

Closes gitaly#144 (closed)