Improve downstream pipeline trigger class

What does this MR do?

This modifies script/trigger-build to:

• Pass a new TOP_UPSTREAM_TRIGGER_PROJECT environment variable used in omnibus-gitlab to post a comment on the CE/EE commit under test
  • The reason why we introduce a new variable is that the multi-pipeline chain goes from gitlab-{ce,ee} to omnibus-gitlab to gitlab-qa and we'll need to have a reference to gitlab-{ce,ee} in the final QA job in order to post the final comment on the CE/EE commit under test once QA is done
• I've also improved/refactored a bit the classes in this file to better differentiate between the trigger tokens and the API tokens

What are the relevant issue numbers?

Second part of https://gitlab.com/gitlab-org/gitlab-ce/issues/47818.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides