Add v2 to reserved top level routes

What does this MR do?

This will add v2 as reserved root level path. The reason why we need this is because we want commands like docker pull work with Rails app. No matter what URL you provide to docker cli it will always add v2 prefix.

For example:

docker pull example.com/foo/bar => GET example.com/v2/foo/bar/manifest

EE version of this MR

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10422

What are the relevant issue numbers?

Its necessary for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9750

Questions:

Do I need a changelog for this?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer