Add an HA validation job to .gitlab-ci.yml

Add a manually triggered task to validate a newer version of GitLab against an HA instance.

Requirements

• Use #3394 (closed) and #3393 (closed) to spin up an HA instance
• Run gitlab-qa against the instance
• Save instance logs as artifacts
• Tear down the instance

Blocked by #3393 (closed) and #3394 (closed)