Description of problem:
When running the Octavia playbook to configure it for a multi-DCN environment the playbook doesn't add a subnet to a router

Version-Release number of selected component (if applicable):
17.1

How reproducible:
100%
- deploy Edge/DCN environment
- run the Octavia playbook

Steps to Reproduce:
1. deploy Edge/DCN environment
2. run the Octavia DCN playbook
3. check the interfaces_info of the lb-mgmt-router, it doesn't contain lb-mgmt-subnet

Actual results:
network is not correctly configured

Expected results:


Additional info:

to run the playbook we need:

create an octavia-dcn-parameters.yaml file with:

octavia_controller_availability_zone: az-central
octavia_availability_zones:
  az-central: # no cidr needed, it uses the already existing subnet
  az-dcn1:
    lb_mgmt_subnet_cidr: 172.47.0.0/16
  az-dcn2:
    lb_mgmt_subnet_cidr: 172.48.0.0/16
octavia_backbone_tenant_cidr: 172.49.0.0/16

$ ansible-playbook -i overcloud-deploy/central/config-download/central/tripleo-ansible-inventory.yaml /usr/share/ansible/tripleo-playbooks/octavia-dcn-deployment.yaml -e @octavia-dcn-parameters.yaml -e stack=central -v
[..]
TASK [octavia_overcloud_dcn_config : Add subnets to lb-mgmt routers] ***********                                                                                                               
changed: [undercloud] => (item=az-central) => {"ansible_loop_var": "item", "changed": true, "cmd": "subnet_id=$(openstack subnet show -f value -c id lb-mgmt-subnet)\nif ! openstack router show lb-mgmt-router -f json -c interfaces_info |  jq -r \".interfaces_info[].subnet_id\" | grep -q $subnet_id; then\n  openstack router add subnet    lb-mgmt-router    lb-mgmt-subnet\n  echo \"subnet added\"\nfi\n", "delta": "0:00:09.158506", "end": "2023-03-07 09:05:47.106511", "item": "az-central", "msg": "", "rc": 0, "start": "2023-03-07 09:05:37.948005", "stderr": "BadRequestException: 400: Client Error for url: https://overcloud.redhat.local:13696/v2.0/routers/209e7fe1-6661-41e6-aecd-8ae1be545dfe/add_router_interface, Bad router request: Subnet for router interface must have a gateway IP.", "stderr_lines": ["BadRequestException: 400: Client Error for url: https://overcloud.redhat.local:13696/v2.0/routers/209e7fe1-6661-41e6-aecd-8ae1be545dfe/add_router_interface, Bad router request: Subnet for router interface must have a gateway IP."], "stdout": "subnet added", "stdout_lines": ["subnet added"]}
[..]

$ openstack router show lb-mgmt-router
+-------------------------+-----------------------------------------------------------------------------------------------------------------------------------------+
| Field                   | Value                                                                                                                                   |
+-------------------------+-----------------------------------------------------------------------------------------------------------------------------------------+
| admin_state_up          | UP                                                                                                                                      |
| availability_zone_hints | az-central                                                                                                                              |
| availability_zones      | az-central                                                                                                                              |
| created_at              | 2023-03-07T09:05:24Z                                                                                                                    |
| description             |                                                                                                                                         |
| external_gateway_info   | null                                                                                                                                    |
| flavor_id               | None                                                                                                                                    |
| id                      | 209e7fe1-6661-41e6-aecd-8ae1be545dfe                                                                                                    |
| interfaces_info         | [{"port_id": "d6dfa89b-ed80-4caa-afc4-d7b676466f6d", "ip_address": "172.49.1.92", "subnet_id": "2f395dcc-9165-4e6a-8e7b-db932a3e9fb4"}] |
| name                    | lb-mgmt-router                                                                                                                          |
| project_id              | f106f40d73694811b73cf25146ab11cc                                                                                                        |
| revision_number         | 3                                                                                                                                       |
| routes                  | destination='172.47.0.0/16', gateway='172.49.0.146'                                                                                     |
|                         | destination='172.48.0.0/16', gateway='172.49.1.71'                                                                                      |
| status                  | ACTIVE                                                                                                                                  |
| tags                    |                                                                                                                                         |
| updated_at              | 2023-03-07T09:11:08Z                                                                                                                    |
+-------------------------+-----------------------------------------------------------------------------------------------------------------------------------------+
$ openstack subnet list | grep 2f395dcc-9165-4e6a-8e7b-db932a3e9fb4
| 2f395dcc-9165-4e6a-8e7b-db932a3e9fb4 | lb-mgmt-backbone-subnet | 9c6871e0-894f-4b8f-9319-8aff958c0c2f | 172.49.0.0/16  |

it should have had a interface on lb-mgmt-subnet


as a workaround we can configure a gateway on the lb-mgmt-subnet before running the playbook.

$ openstack subnet set --gateway 172.24.0.1 lb-mgmt-subnet