Geo: Implement alternative to geo_{primary|secondary}_role in gitlab.yml

What does this MR do?

Reverts the requirement and use of geo_{primary|secondary}_role in gitlab.yml to run Geo.

Are there points in the code the reviewer needs to double check?

Double check if the problematic parts (see related MRs / Issues here: gitlab-org/gitlab-ee#2826) are still working as expected

Why was this MR needed?

The addition of the roles in gitlab.yml were a tentative to fix many different issues. But this adds complexity to the code and requires yet another step/configuration from the user.

This MR reverts some of the changes implemented in previous MRs and use a simple solution instead.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2826 (closed)

cc @stanhu @dbalexandre