Comment 4 for bug 1920982

From my testing with Ussuri, `enable_isolated_aggregate_filtering` does not need to be enabled in order for `limit_tenants_to_placement_aggregate` to work. These are two entirely separate settings (explained succinctly below):

- `limit_tenants_to_placement_aggregate` is used to place instances of particular tenants onto particular aggregates, and is detailed in [1] and [2]. The filtering is performed using project IDs.

- `enable_isolated_aggregate_filtering` is used to ensure that *only* instances matching particular traits (whether by flavour or image metadata) can be scheduled to hosts in a particular aggregate. With this set to False, instances without these traits can also be scheduled to hosts in the aggregate. The filtering is done with traits. See [3] and [4].

Note that `placement_aggregate_required_for_tenants` [5] requires that *all* tenants be confined to an aggregate (i.e. they will need to go through the steps detailed in [1] for every tenant). Presumptuously, this is probably not the behaviour that most users want or expect; they probably want to limit a single host or set of hosts to a particular tenant but otherwise allow instances to be freely scheduled. In this case, using `enable_isolated_aggregate_filtering` and following [3] (i.e. using traits) I think is the best approach.

Despite all the above, I see that both this bug and [6] mention `enable_isolated_aggregate_filtering` and `limit_tenants_to_placement_aggregate` together. Hopefully I'm not missing something here. If I haven't, the patch needs to be rejected in order to separate these two settings out.

[1] https://docs.openstack.org/nova/ussuri/admin/aggregates.html#tenant-isolation-with-placement
[2] https://docs.openstack.org/nova/latest/configuration/config.html#scheduler.limit_tenants_to_placement_aggregate
[3] https://docs.openstack.org/nova/latest/reference/isolate-aggregates.html
[4] https://docs.openstack.org/nova/latest/configuration/config.html#scheduler.enable_isolated_aggregate_filtering
[5] https://docs.openstack.org/nova/ussuri/configuration/config.html#scheduler.placement_aggregate_required_for_tenants
[6] https://bugs.launchpad.net/charm-nova-cloud-controller/+bug/1908740