Protected Environments follow up: Use class method to override CE constants on EE

What does this MR do?

Protected Environments follow up: Use class method to override CE constants on EE. Original discussion here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6672#note_93676593

CE port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21545

What are the relevant issue numbers?

#7204 (closed)

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
• [-] EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?