Fix the QA image names in QA CI jobs

What does this MR do?

The QA image name was wrongly set from the project name, which isn't gitlab-ce/gitlab-ee anymore. This fixes that by using another strategy instead (check for the existence of the ee/ folder).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Closes gitlab-qa#406 (closed).