Description of problem:

When scaling down a compute node after scaling up an IPv6 deployment, the operation fails

Version-Release number of selected component (if applicable):
OSPd-7
python-rdomanager-oscplugin-0.0.10-28.el7ost

How reproducible:

This happens with every type of node we're trying to scale down (tried both compute and ceph)

Steps to Reproduce:
1. Deploy with IPv6
2. Scale up one node
3. Scale down one node

Actual results:

Update fails, node is still present

Expected results:

Node is removed

Additional info:

The command

openstack overcloud node delete 64108a1d-b8a4-4680-afa1-e8d41059cccf --debug --templates /home/stack/local_tht/ --stack overcloud -e /usr/share/openstack-tripleo-heat-templates/environments/network-isolation.yaml -e /usr/share/openstack-tripleo-heat-templates/environments/net-single-nic-with-vlans.yaml -e ~/network-environment.yaml -e /usr/share/openstack-tripleo-heat-templates/environments/storage-environment.yaml

returns successfully

but a heat stack-list immediately returns UPDATE_FAILED

heat resource-list overcloud returns:

 Networks                                  | b31470b8-7230-4b4b-aabc-580700448a90          | OS::TripleO::Network                              | UPDATE_FAILED

heat event-list overcloud returns 

| Networks                                  | 657d4a7f-1030-4124-94b6-550e9c6134a9 | resources.Networks: resources.StorageMgmtNetwork: Conflict: resources.StorageMgmtSubnet: Unable to complete operation on subnet 5b85e3dd-5502-41f2-bb8f-367a8fc3b52c. One or more ports have an IP allocation from this subnet. | UPDATE_FAILED 

neutron net-list

9d5921f9-ba08-4147-8809-9ffb424f8cb0 | storage_mgmt | 5b85e3dd-5502-41f2-bb8f-367a8fc3b52c fd00:fd00:fd00:4000::/64

neutron port-list

| bf9ce797-52bb-4bd1-88b6-e3c4585c2be7 |                               | fa:16:3e:c7:16:e4 | {"subnet_id": "5b85e3dd-5502-41f2-bb8f-367a8fc3b52c", "ip_address": "fd00:fd00:fd00:4000::12"} |