OpenStack provides a feature called scheduler hints that allow a user to inform the scheduler where they desire their instance to run. 

Policies available to use:

• Affinity (same host using server groups)
• Anti-affinity (different hosts using server groups)
• DifferentHost (using instance UUIDs)
• SameHost (using instance UUIDs)
   

openstack server create --image IMAGE_ID --flavor 1 --hint group=SERVER_GROUP_UUID server-1

openstack server create --image IMAGE_ID \
  --flavor 1 --hint different_host=INSTANCE_ID_1 \
  --hint different_host=INSTANCE_ID_2 MY_THIRD_INSTANCE

More documentation on how to use these filters is available upstream at: https://docs.openstack.org/ocata/config-reference/compute/schedulers.html#servergroupaffinityfilter