Ensure consistency of snap versions across application units

[James Page]

For multi-unit deployments (local or MAAS mode) we need to ensure that the version of the snap being used in a deployment for a specific snap is consistent on all units.

We can achieve this to an extent by using cohorts:

https://forum.snapcraft.io/t/managing-cohorts/8995

I think we should be using these for openstack-hypervisor and microceph snaps as driven by their respective charm; this will most likely be superseeded longer term when we can tie a specific snap revision and charm revision together in the charmhub, improving consistency and reproducability of a deployment further.