Move setup of Gitaly int MTLS to reusable component
After https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/1077 is ready we should have a reusable Gitaly component.
As https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/1077#note_1191681645 points out the MTLS scenario could be modified to make use of this Gitaly component.