Perhaps I do not fully understand your task but Tenant is logical unit that completely isolated.
No tenant can access the instance of another tenant, even though the software might be running on the same machine (hence the logical isolation).
Therefore tenants have isolated user databases and all settings from my point of view.
In that case you can create 7th tenant like master-tenant with all users, roles and metadata but this looks too complicated for me.
P.S. I am using different tenants for Production and Staging, with multi-applications in every Tenant.