Bump VERSION to 4.15.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.14.0...71abb03ce6f6c1931ae5fa2596589b36912e457b

• Release notes:

### New scenarios and scenario updates

- !345 Add LFS object storage config to object_storage scenario
- !344 Don't require GCLOUD_REGION for Kubernetes

• 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:

  • Create a tag for the new release version.