Update gitlab-qa version to 9.1.2

Diff

• https://gitlab.com/gitlab-org/gitlab-qa/compare/v9.1.1...6ddad73cb59f05b220f4b57f34d8ad5ac825a2fc

Release notes

Fixes

• Switch object storage proxy_download setting fr... (!1135 - merged)
• Use db:schema instead of db:structure rake task (!1137 - merged)

Checklist before merging

• Diff link is up-to-date.
• Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
• Release notes are accurate.

Checklist after merging

• Update the release notes for the newly created tag.