This adds upgrade testing from our current Gerrit version (3.5) to the
likely future version of our next upgrade (3.6).
To do so we have to refactor the gerrit testing becase the 3.5 to 3.6
upgrade requires we run a command against 3.5. The previous upgrade
system assumed the old version could be left alone and jumped straight
into the upgrade finally testing the end state. Now we have split up the
gerrit bootstrapping and gerrit testing so that normal gerrit testing
and upgrade testing can run these different tasks at different points in
the gerrit deployment process.
Now the upgrade tests use the bootstrapping playbook to create users,
projects, and changes on the old version of gerrit before running the
copy-approvals command. Then after the upgrade we run the test assertion
portion of the job.