Bump version to 4.18.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.17.0...b42e87d3f7b9405d7f99c4058ac6536ca74335dc

• Release notes:

### Fixes

- !370 Don't include the variable names

### Doc changes

- !368 Update run_qa_against_gdk.md

### Other changes (CI, backstage)

- !369 Add parallel version of custom job
- !371 Test Elasticsearch version 6.4.2
- !372 Enable all object storage settings
- !374 Add ability to collate per-stage reports

• 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.