Today Cinder's API design assumes that a project_id will be on every request[1].

In order to consume system scoped requests, Cinder will need to update its APIs to remove project_id from the path. For example, use:

https://cinder.example.com/v3/volumes

instead of:

https://cinder.example.com/v3/422c276436a44096bb7cd626ef1813ca/volumes

where 422c276436a44096bb7cd626ef1813ca is the id of a project in keystone.

[1] - https://docs.openstack.org/api-ref/block-storage/v3/index.html#volumes-volumes