Extension of the tenant management

[yclausen]

Currently, only the owner of a tenant can update, delete and add new users to the tenant in tenant-manager.
That means you always have to know the login data of all tenant owners if you want to delete or update a tenant.

It would be nice if the tenant-manager offers the possibility that an owner of a tenant can give another user the permission to also edit (add and delete a user to tenant) or delete the tenant.
This would also improve and simplify the process of API access from the tenant-manager.