If the instances per host are not cached in the HostManager
we lookup the HostMapping per candidate compute node during
each scheduling request to get the CellMapping so we can target
that cell database to pull the instance uuids on the given host.
For example, if placement returned 20 compute node allocation
candidates and we don't have the instances cached for any of those,
we'll do 20 queries to the API DB to get host mappings.
We can improve this by caching the host to cell uuid after the first
lookup for a given host and then after that, get the CellMapping
from the cells cache (which is a dict, keyed by cell uuid, to the
CellMapping for that cell).
Change-Id: Ic6b1edfad2e384eb32c6942edc522ee301123cbc
Related-Bug: #1737465
(cherry picked from commit 3a69fdb333)
677c95280b