Run the tests against a genuine Gitaly service

The following discussion from !300 (merged) should be addressed:

• @nick.thomas started a discussion:

  It's a Gitaly, rather than SSH, server, so I'd rename this.

  Instead of, or even in addition to, this, we could consider using a genuine Gitaly service to run the tests against, in the same way we do for gitlab-workhorse and gitlab-elasticsearch-indexer: https://gitlab.com/gitlab-org/gitlab-workhorse/blob/master/.gitlab-ci.yml#L8 and https://gitlab.com/gitlab-org/gitlab-workhorse/blob/master/gitaly_integration_test.go#L54 . WDYT?