Make uploaded invoice file name configurable and change to `stack`

https://github.com/CCI-MOC/openstack-billing-from-db/blob/29fc01f5151ed1abdeead6a477a9425b85523037/src/openstack_billing_db/billing.py#L270

Currently it uses `NERC OpenStack`, but the name should
- be configurable
- be the internal cluster name