Fix setup of repository storages for the test environment

What does this MR do?

On the backup_rake_spec we require a second repository storage, which we used to accomplish through a combination of rspec-mocks and special configurations in the Gitaly installation. Since Gitaly now refuses to start with invalid storages, we modified the setup code in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22530, but in between tests the storage could be removed from the tmp/ folder.

These changes simplify the approach by making the secondary storage a real storage and having TestEnv manage it the same way it does the default storage.

/cc @zj

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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 e2 tests" label. See the Test Planning Process.