Implement multiple repository mount points

What does this MR do?

Refactor repository paths handling to allow multiple git mount points

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

Pretty much all of it.

Why was this MR needed?

Mostly because we need the ability to have multiple git mount points to distribute storage load and make it easier to migrate to new storage shards.

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ee/issues/583

Screenshots (if relevant)

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