Check out this link that explains the difference between Availability Zones and Host Aggregates. https://www.mirantis.com/blog/segregation-in-grizzly-availability-zones-versus-host-aggregates/

According to Opentack documentation docs.openstack.org maybe you are looking for is called " Availability Zones".

To define / assign a node to an avalibity zone, follow the directions here

node_availability_zone=availability_zone_name
service openstack-nova-metadata-api restart
service openstack-nova-compute restart

If you launch your VM from Horizon Dashboard you can select the availability zone from a drop down menu.

If you launch an instance from the command line you can specify which availability zone to use by name here.