In order to facilitate a future extraction of the placement service
we want to record the association between a resource provider and an
arbitrary aggregate uuid in its own table.
A PlacementAggregate model is joined from ResourceProvider via
ResourceProviderAggregate. Note that this structure is used so we can
join on ids instead of strings (the uuids). A direct mapping between
ResourceProvider uuid and Aggregate uuid was mooted earlier in the year
but was determined to be suboptimal.
The name 'placement_aggregates' is used as the least problematic of
several choices after discussion amongst several parties.
The data will be used by the forthcoming get_ and set_aggregates
methods on the ResourceProvider object.
Change-Id: Id0355cb022f68e962af306ff04cf724d22b68d19
Partially-Implements: blueprint generic-resource-pools-ocata
320f60f28b