domain list via projects api with domain-scoped token is always empty

[Boris Bobrov]

Listing domains via projects api (/v3/projects) using is_domain parameter with domain-scoped token always returns an empty list.

Steps to reproduce:

1. Get a domain-scoped token
2. Make a call to /v3/projects?is_domain=true with the token

Expected:
Domains are listed (given the policies allow it). Or i get an error message that it is impossible to list is_domain projects with a domain-scoped token.

Observed:
Domain list is empty.

Probable reason:
https://opendev.org/openstack/keystone/src/commit/1e7ecca881a51144d61ae8026e1a77d6669997e2/keystone/api/projects.py#L135-L139 - with domain-scoped token projects are filtered by domain_id. Domains have no domain_id and are filtered out.

How it was discovered:
Terraform OpenStack Provider does not use /v3/domains endpoint to fetch information about domains. Instead, /v3/projects is supposed to be used. https://github.com/terraform-provider-openstack/terraform-provider-openstack/tree/32f312ff538b846c32b93247f94c58163a6145f1/openstack

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/keystone/+/844637