Show clear message when set-geo-primary-node was successful

What does this MR do?

Make sure it's clear when rake set-geo-primary-node was successful.

Screenshots (if relevant)

$ rake set-geo-primary-node
Saving primary Geo node with URL https://primary.geo.example.com/ ...
https://primary.geo.example.com/ is now the primary Geo node

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] 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

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#2177.