Fleet usage data in virtual_provisioning_collection should be labeled as rack usage

[askfongjojo]

While looking at #6653, we notice that the entry with collection_type = fleet in the virtual_provisioning_collection database table should really be labeled as rack. The distinction is important as the numbers are consumed in metrics. If an external system already collected the rack usage as fleet usage, we will have to make up another collection type to aggregate multiple racks in the future and it's potentially confusing.