Add e2e test of push over SSH using Git protocol v2

What does this MR do?

Adds an end-to-end test of pushing over SSH using Git protocol v2

This won't work in the nightly or package-and-qa pipelines until the omnibus images support Git protocol v2 so I've added a flag to indicate whether the environment supports the test (gitlab-qa!198 (merged)). The flag has been set to skip the test in nightly and package-and-qa pipelines (see https://gitlab.com/gitlab-org/gitlab-ce/issues/53075). I.e., it will only run in staging for now.

What are the relevant issue numbers?

• Test plan: https://gitlab.com/gitlab-org/gitlab-ce/issues/51236
• Feature issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/46555

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