upgrade-charm results in hook failed: "config-changed"

[Jake Hill]

Upgrading an old keystone-296 (bionic-stein) to the latest (317) produces the following error;

unit-keystone-0: 13:29:40 WARNING unit.keystone/0.config-changed raise RuntimeError('Leader unit has not provided credentials required '
unit-keystone-0: 13:29:40 WARNING unit.keystone/0.config-changed RuntimeError: Leader unit has not provided credentials required for speaking with Keystone yet.
unit-keystone-0: 13:29:40 ERROR juju.worker.uniter.operation hook "config-changed" (via explicit, bespoke hook script) failed: exit status 1

keystone/0 is the leader (it is a single instance).