Fix the QA image retrieval for staging

This implements https://gitlab.com/gitlab-org/gitlab-qa/issues/398 and provide a more robust solution to https://gitlab.com/gitlab-org/quality/staging/issues/56.

TODO:

• Replace GITLAB_QA_DEV_ACCESS_TOKEN value with a valid access token for the gitlab-qa-bot user on dev. I've used my own access token for now. It seems the dev password for this user stored in 1Password is wrong, and the current GITLAB_QA_DEV_ACCESS_TOKEN token set for the https://gitlab.com/gitlab-org/quality/staging doesn't have sufficient scopes. => I removed the variable and both GITLAB_QA_ACCESS_TOKEN and GITLAB_QA_DEV_ACCESS_TOKEN variable should be given manually when triggering the staging job.

Also, I've opened https://gitlab.com/gitlab-org/quality/staging/issues/71 for the broken API token and wrong password for the gitlab-qa-bot user.

Closes https://gitlab.com/gitlab-org/gitlab-qa/issues/398, https://gitlab.com/gitlab-org/release/framework/issues/421.