Update Gitaly n+1 tests and Sitespeed to use the onprem testbed
WIP: Depends on gitlab-org/gitlab-qa!243 (closed) being merged first.
I'm trying to remove the use of preprod and make the Gitaly n+1 and Sitespeed tests use the new onprem environment instead.
I haven't contributed to this project in a meaningful way yet, so please review closely and all feedback is very welcome!
Edited by Tanya Pazitny