Execute FDW SystemChecks only in Geo Secondary node

What does this MR do?

FDW checks should only run in Geo secondary nodes

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

FDW check is new behavior in 10.6, so we need to fix this for 10.6 before release.

Why was this MR needed?

When running it in a primary node, FDW checks would fail with exception instead of being skipped

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary not necessary as this will still be part of 10.6.0
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5266 (closed)

cc @mkozono @vsizov