I had this question a couple of days back and searching around found
this nice explanation:
http://blog.russellbryant.net/2013/05/21/availability-zones-and-host-aggregates-in-openstack-compute-nova/
So yes, if you create a new host aggregate, and pass an AZ name to it,
users can use that to boot
Trying to get a good idea of how they work. As I get it, host
aggregates are collections of (not necessarily mutually exclusive) hosts
that share a metadata value. And they can be part of availability zones
which can be (say) specific to networking hardware, or CPU, or
something. What I'm tr