Enable Gitaly by default in GitLab 9.3

What does this MR do?

Change defaults so that gitlab-ce/gitlab-workhorse etc. use Gitaly, and so that the init script starts the Gitaly service.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We want Gitaly to become a mandatory component of GitLab. This is the next step towards that goal.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitaly/issues/242