join-up.sh unavailable when unknown bootenv differs from discovery-base

[kapsh]

Steps to reproduce

1. drpcli prefs set unknownBootEnv ignore (reasons below)
2. Try to add existing machine into drp as suggested in https://provision.readthedocs.io/en/latest/doc/content-packages/drp-community-content.html#discovery
   curl -fsSL [internal ip]:8091/machines/join-up.sh

Expected result

join-up.sh is fetched.

Actual result

Error 404 which is confusing. Default bootenv requirement for that is unclear for user and doesn't mentioned anywhere in the docs.

[kapsh]

Rationale

My desired workflow is to manage only known set of machines with DRP. They coexist in the same subnet with multiple machines that shouldn't be interfered with. So I'm trying to add machines only using join-up.sh while ignoring everything else: when "my" machine reboots after workflow change, it will rediscover itself; when someone else machine boots in pxe (mostly accidentaly) it won't appear in DRP console and won't baffle machine owner.

Am I holding it wrong? Probably this can be fixed with activestart/activeend subnet options, but this will require moving my machines into another subnet.

PS Maybe this issue should be in provision-content repo.