What you are asking, is an "Availability Zones" .

See below description:

Host aggregates are a mechanism to further partition an availability zone; while availability zones are visible to users, host aggregates are only visible to administrators. Host Aggregates provide a mechanism to allow administrators to assign key-value pairs to groups of machines. Each node can have multiple aggregates, each aggregate can have multiple key-value pairs, and the same key-value pair can be assigned to multiple aggregates. This information can be used in the scheduler to enable advanced scheduling, to set up hypervisor resource pools or to define logical groups for migration.

For more details check out here: http://docs.openstack.org/havana/conf...

Good luck!