Don't allow to specify a repo or version when installing Workhorse
As pointed out by @rspeicher in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6574#note_19266284, the GITLAB_WORKHORSE_VERSION is not necessary, the task should always use the required GitLab Workhorse version.
With the same logic, I've removed the option to specify a different repository (with the GITLAB_WORKHORSE_REPO env variable), again, the task should use the canonical one. The task is not meant to be used for development/debugging purpose.
I've also updated the "patch version upgrade" doc since I did forget to update it in !6574 (merged). :) /cc @jacobvosmaer-gitlab