Our code in https://github.com/theforeman/foreman/blob/develop/app/models/compute_resources/foreman/model/openstack.rb#L65 uses :display_name as the name attribute to create Openstack volumes.
Notice volume v2 API requires :name, while v1 just needs :display_name
https://github.com/fog/fog/blob/master/lib/fog/openstack/models/volume_v1/volume.rb
https://github.com/fog/fog/blob/master/lib/fog/openstack/models/volume_v2/volume.rb#L17

So we cannot create Volumes on Openstack when they use the v2 api. Fix is simple, just add :name in that hash and it works.