PR: https://github.com/truenas/middleware/pull/6573

A not-insignificant number of users will first try deploying TrueNAS
in a lab / test domain before migrating to the final production domain.
This may be done haphazardly without cleanly leaving the old AD domain.
To ease the migration process, we should clear out middleware user
cache when the AD service is explicitly stopped and flush samba caches.