Geo: Added extra SystemCheck checks

What does this MR do?

Adds extra SystemChecks to make sure secondary nodes are correctly configured.

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

Why was this MR needed?

Configuring Geo requires lots of steps, if we can detect ideogrammatically a missing step, it's easier to let the user fix.

This will implement few items from gitlab-org/gitlab-ee#727

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 #2860 (closed), gitlab-org/gitlab-ee#2469

cc @stanhu @dbalexandre