Allow Gitaly to be built from a custom URL

What does this MR do?

When running the test suite, this respects a new environment variable: GITALY_REPO_URL. If set, then we'll try to run the tests with a Gitaly cloned from that URL, rather than using the canonical https://gitlab.com/gitlab-org/gitaly.

The idea behind this is that people running a fork of gitlab-ce can point at their fork of gitaly using CI variables. By including an access token in the URL, this allows the gitlab-ce jobs to clone from a private Gitaly repository. Super-useful.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer