Add simple identifier to public SSH keys

## What does this MR do?
Adds a simple identifier of user_name + hostname to public SSH keys

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

## Why was this MR needed?
To help people identify keys when they export them

## What are the relevant issue numbers?
#18866 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ -] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ -] API support added
- Tests
  - [ x] Added for this feature/bug
  - [ x] All builds are passing
- [ x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #18866

See merge request !5614