Limiting Placement Allocation Candidates

In a large and sparse cloud (e.g. 10,000 empty compute nodes), a request of GET /allocation_candidates?resource=VCPU:1 can return information for 10,000 resource providers. This has implications for memory and performance on both the placement service and in the client (in the present day, the nova-scheduler). To get around these concerns a 'limit' is proposed. A naive implementation of 'limit' will impact the pack or spread style of the cloud as the database will have a "natural" order that will put certain hosts first. To get around this an indexable column of randomness can be added to the db and the limited query can be sorted on that. If the values of that column are set only once, the cloud is biased to pack. If the values are updated regularly on a periodic job, the cloud is biased to spread.