Override CI_COMMIT_REF_SLUG for QA branches
Override CI_COMMIT_REF_SLUG for QA branches

The `gitlab:assets:compile` job isn't run for the QA branches, thus
there's no Docker image correspinding these branches in the registry.

By overriding `CI_COMMIT_REF_SLUG` to `master` for QA branches, the
`fetch-assets` job in the `omnibus-gitlab` pipeline will pull the
`master` assets Docker image.

Signed-off-by: Rémy Coutable <4ea0184b9df19e0786dd00b28e6daa4d26baeb3e@rymai.me>

dreampet/gitlab,stoplightio/gitlabhq,dreampet/gitlab,iiet/iiet-git,dreampet/gitlab,axilleas/gitlabhq,axilleas/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,iiet/iiet-git,iiet/iiet-git,mmkassem/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,stoplightio/gitlabhq