Standardise on self-signed certificates for PostgreSQL TLS

What does this MR do?

Removes "use existing certificates" as an option from the documentation. Too much choice is confusing!

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

This MR depends on automatic certificate generation being supported in Omnibus: omnibus-gitlab!2101 (merged)

Why was this MR needed?

Simplify the Geo setup process

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Has been reviewed by Documentation
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Related to omnibus-gitlab#2925 (closed)