Scaling Down Replicas does not take into account pool topology

[tiagolobocastro]

Describe the bug
Although modifying a volume topology is not currently available as a feature, we have suggested to a user modifying etcd under the hood.
Unfortunately when scaling down a volume replica count may remove first replicas which don't match the new volume topology.

To Reproduce

1. Create pools with no topology and other pools with topology labels a=b
2. Create 2-replica volume with no pool topology labels
3. Publish volume
4. Modify volume to add pool topology labels a=b
5. Scale volume to 3 replicas
6. Wait for rebuild
7. Scale down to 2 replicas
   • Here it doesn't always work

Expected behavior
All else being equal, we should always prefer to remove replicas from pools with topology mismatched with volume.