Build Heptapod Workhorse and Shell

Once heptapod-docker#10 (closed) is done, the Heptapod packaging process as a whole will first have Omnibus Heptapod build the upstream GitLab Workhorse and GitLab Shell in Omnibus Heptapod, then Heptapod Docker build their Heptapod counterparts.

Building Heptapod Workhorse directly from Omnibus is a step towards the ultimate goal of Heptapod Omnibus providing fully functional packages and the final Docker images.

As an immediate benefit, that will save us a couple of minutes and some download bandwidth in the CI. It will also make the final image about 50MB smaller.

But we'll need either to implement some kind of HgFetcher or to have the CI of both projects create tarballs, with the same kind of special cases about version numbering that we've already had with Heptapod Rails.

In all cases, we must decide where to store the versions for these components in the Omnibus Heptapod repo (new files? change of value for GITLAB_WORKHORSE_VERSION?)