[2/n] [reconfigurator] soften blippy note for nexus generation mismatch

[sunshowers]

Found while working on property-based/chaos tests for the reconfigurator.

Previously, when two Nexus instances were at the same generation but had different image sources, we would emit a blippy FATAL note. But it is possible for the system to see such a state around MUPdate. With this change:

• if two Nexus instances have the same generation but different Artifact image sources, we still produce a FATAL note
• but if there's just one Artifact version and one InstallDataset, we now produce a less severe note

Also add some more unit tests to blippy.

See inline comments and the new reconfigurator-cli script for more details.

TODO:

• At the moment it looks like the only less-than-fatal severity is BACKCOMPAT, and I've reused that. But I don't think that's actually correct -- we should probably have a WARN severity that sits in between FATAL and BACKCOMPAT.