sled-agent should not bounce zones if the image source changes but their hashes match

[sunshowers]

I'm not sure we'll need to do this here; the normal upgrade path should change zone sources already, right? (It just needs to not do that while a mupdate override is in place.)

Although maybe sled-agent should do something like "if I'm changing from install dataset with hash X to artifact with hash X, don't actually bounce the zone".

Originally posted by @jgallagher in #8456 (comment)