Comment 7 for bug 1679750

As the commit message in Mel's test patch points out:

"There might still be a problem if the compute host is never brought
back up, as allocations will still exist for the instance and show
up as usage during usage queries to placement."

If the compute is brought back up and "heals" the allocation, things are OK, but if the compute is never brought back up it'll be a problem as the GET /usages call to placement would be wrong, and we plan on eventually using that for our quota/limit checks in the API if a cell is down.