Make gitlab-shell go through Workhorse

From https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7168#note_193976203, we discovered that gitlab-shell talks directly to Unicorn instead of Workhorse. As a result, we don't get the queue_duration field in the logs to track how long the request sat in the queue before being processed. In /var/opt/gitlab/gitlab-shell/config.yml, my test instance has:

# Url to gitlab instance. Used for api calls. Should end with a slash.
gitlab_url: "http://127.0.0.1:8080"

I changed it to this to make it work:

gitlab_url: 'http+unix://%2Fvar%2Fopt%2Fgitlab%2Fgitlab-workhorse%2Fsocket'

@jacobvosmaer-gitlab Is there a reason we shouldn't make this the default?