After the cellification of the Aggregates API, we introduced a requirement
that the service must have a HostMapping record, so that we know which
cell the service is in. This is normally fine, but for weird drivers such
as ironic, there may be valid cases (i.e. during setup) where no ironic
nodes are present, thus one or more services may not have any compute node
records, and thus cannot be added to aggregates.
This adds a cell scan, only if necessary, to find the desired service so
that the operation may proceed as it did before. To do this, we refactor
the _find_service() helper to a more generic utility and use that if we
don't find a HostMapping during the add operation.
Change-Id: Idc97126d63684e7d638b974d7226ff210c744404
Closes-Bug: #1686744
62878ef5a6