Code review comment for ~cjwatson/launchpad:build-tarball

Swift is OpenStack's distributed object store component. See https://docs.openstack.org/swift/latest/ for documentation. In PS5 the same API is provided by ceph-radosgw (https://docs.ceph.com/en/latest/radosgw/index.html), but it ends up being much the same. We use it for the Launchpad librarian, but it's also a handy way to store blobs for deployment.

Juju charms will indeed eventually let us refactor a whole bunch of confusing deployment machinery, but they don't help with the preliminary step of building code artifacts for deployment; that still has to be done separately one way or another. This work will replace something that's currently done by a combination of carob:~pqm/bin/update-launchpad-built-code.sh (which builds a tree) and lp:deployment-manager (which among other things rsyncs that tree out to target machines), as well as some even more confusing and less coherent things on (qa)staging.