Zuul is publishing lovely container images, so we should
go ahead and start using them.
We can't use containers for zuul-executor because of the
docker->bubblewrap->AFS issue, so install from pip there.
Don't start any of the containers by default, which should
let us safely roll this out and then do a rolling restart.
For things (like web or mergers) where it's safe to do so,
a followup change will swap the flag.
Change-Id: I37dcce3a67477ad3b2c36f2fd3657af18bc25c40
