Mirror nova host aggregates in placement

Placement aggregates are not the same thing as nova host aggregates. Placement aggregates are merely groups of associated resource providers. Nova host aggregates are collections of `nova-compute` service workers (hypervisor hosts). Nova host aggregates can have metadata associated with them. Placement aggregates have no metadata associated with them (the resource provider is the thing that has this metadata associated with it).

We still, however, need a mechanism of populating the placement_aggregates table with a record for each nova host aggregate if we are going to take advantage of the grouping/association knowledge in placement queries.