Using checks of `,$PUPPET_CLASSES,` is repetitive and error-prone; it
does not properly deal with `zulip_ops::` classes, for instance, which
include the `zulip::` classes.
As alluded to in ca9d27175b, this can be fixed by inspecting the
classes that would be applied, using `puppet --write-catalog-summary`.
We work around the chicken-and-egg problem alluded to therein by
writing out as complete `zulip.conf` as would be necessary, before
running puppet and removing the sections we then know to not be
needed.
Unfortunately, there are two checks for `$PUPPET_CLASSES` which cannot
be switched to this technique, as they concern errors that we wish to
catch quite early, and thus before we have puppet installed. Since we
expect failures of those to only concern warnings, and only be
mistakenly omitted for internal `zulip_ops::` classes, this seems a
reasonable risk to admit in exchange for catching common errors early.
9c0fd632bb