This adds a request filter that, if enabled, allows us to use placement
to select hosts in the desired availability zone by looking up the uuid
of the associated host aggregate and using that in our query for
allocation candidates. The deployer needs the same sort of mirrored
aggregate setup as the tenant filter, and documentation is added here to
make that clear.
Related to blueprint placement-req-filter
Change-Id: I7eb6de435e10793f5445724d847a8f1bf25ec6e3
96f1071166