That particular path, for cloudinit's benefit, is hard-coded all over the provider code. It may be an obstacle to testing, because tests can't substitute a fake (and obviously tests shouldn't mess with /var/lib/juju, or even rely on its existence).
If we later find that one of these mentions is different from the others, it's quite possible that we have to specialize some of the uses into separate variables. For now, the main things are to be aware, and to have an injection point for tests.