Fix Import/Export not working in HA mode

Use a shared path instead of Tempfile default /tmp so the import file is accessible by any GitLab instance.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20506

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together