[QA] Improve the fork scenario to take a username and password instead of always…

What does this MR do?

This allows the fork scenario to take in account GITLAB_FORKER_USERNAME and GITLAB_FORKER_PASSWORD in order to log-in with an existing user to fork the project instead of always creating a new user.

This needs a gitlab-qa MR to forward the GITLAB_FORKER_USERNAME and GITLAB_FORKER_PASSWORD env variables: gitlab-qa!166 (merged)

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/quality/staging#2 (closed).