[12/n] [sled-agent] don't start install dataset zones on mupdate override error

[sunshowers]

For mupdate overrides, in order to be safe, we must know that the data stored in the JSON is consistent with the images stored on disk. We currently apply this logic to both install dataset and artifact hashes, since we need to consult mupdate override info for the

TODO: testing on a racklette, putting this up to get feedback for now

Questions:

• Should we apply this logic to more critical zones like the switch zone? They're always going to be part of the ramdisk maybe?
• How does this interact with config-reconciler work? That would permanently be blocked it seems to me.
• We ask for the boot zpool to be passed in but also cache it as part of constructing the info... how do we reconcile the relative presence or absence of this info?

Depends on:

• [11/n] [sled-agent] move around ZoneImageFileSource logic #8235
• earlier commits in the stack

[sunshowers]

We need to figure out what to do if the mupdate override is not in place (i.e. if it was removed by sled-agent). Presumably the install dataset will still be around in that case.