Gitaly testing toml

What does this MR do?

• Build a config.toml file when runnning the gitlab:gitaly:install task.
• Use that file when running the gitaly instance for the test environment (instead of using ENV variables, which is our current approach)

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

No

Why was this MR needed?

Moving forward on Gitaly the preferred way of specifying configuration is the configuration file

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/182