"cinder-manage db online_data_migrations" does not appear to be handled by charms

During an O7k upgrade from Train to Ussuri, and following a Stein to Train upgrade by only a ouple of days, my attempt to run the openstack-upgrade action on the cinder leader (with action-managed-upgrades=true and both principal and hacluster units paused) met with failure.

I was able to extract this from the errors:

----
Error during database migration:
      Migration cannot continue until all volumes have been migrated to the `__DEFAULT__`
      volume type. Please run `cinder-manage db online_data_migrations`. There are
      still untyped volumes unmigrated.
----

Additionally, the non-charmed upgrade docs for cinder (https://docs.openstack.org/cinder/train/upgrade.html#after-maintenance-window) mention the need to run "cinder-manage db online_data_migrations" as an "after maintenance window" action.

We may want the charm to handle this in some way, or if it automatically handles itself gradually, we may want to add something to the charm deployment guide highlighting this.

Because this occurred, I ended up with a cinder leader upgraded to Ussuri packages but without DB migrations ("cinder-manage db sync") applied. As packages were upgraded, I couldn't re-run the action to apply the upgrade since it thought it was already upgraded; instead, I had to manually run "cinder-manage db sync" to apply the Ussuri migrations.