1.15 upgrade notes don't clearly specify whether to run the container runtime migration steps before or after upgrading

[aieri]

k8s 1.15 switches the default container runtime to containerd. The upgrade notes point out that

You have the option of keeping Docker as the container runtime, but even in that case you must perform the upgrade steps.

It is however not clear if these steps should be run while still on 1.14, after having upgraded to 1.15, or perhaps in between upgrading the master/worker charms and running their upgrade actions.