Allow to pull code with deploy key from public projects

What does this MR do?

With deploy keys you can download any public projects stored in GitLab.

What are the relevant issue numbers?

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/1217

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • 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