Add e2e test: push mirroring over SSH with private key

What does this MR do?

Adds an end-to-end test of push mirroring a repository over SSH using a private key

It currently includes the code from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23086 because that MR makes changes to the framework that the test in this MR depends on. That commit will be removed once that MR is merged.

What are the relevant issue numbers?

gitlab-qa#344 (moved) https://gitlab.com/gitlab-org/gitlab-ce/issues/52576

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 e2e tests label. See the Test Planning Process.