Description of problem:
When trying to prepare for OVN migration according to the official documentation of the latest release, the inventory generated by ovn_migration script is not correct, e.g there are missing nodes in mandatory sections, see [1] below.

Probably this happens because the ansible yaml file retrieved from the OSP17 environment has different format than OSP16 has. See the attached yaml file, generated by this command (similar command runs the ovn_migration script):
/usr/bin/tripleo-ansible-inventory --stack overcloud --static-yaml-inventory /tmp/inventory_file.yaml

Also find below an example of a valid inventory [2]


Version-Release number of selected component (if applicable):
RHOS-17.0-RHEL-8-20211208.n.1

How reproducible:
100%

Steps to Reproduce:
1.Deploy an OSP17.0 environment with ML2/OVS neutron backend
2.Due to BZ2035354 and BZ2035808 do the following workaround:

a)Clone upstream neutron repo stable/wallaby branch (OSP17.0 is based on it) and copy ovn_migration.sh script to /usr/bin and ovn migration ansible playbooks to  ~/ovn_migration (according to the official documentation for the latest OSP version, https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.2/html-single/networking_with_open_virtual_network/index#ml2-ovs-to-ovn-migration-prepare_migrating-ml2ovs-to-ovn), 
i.e.
git clone -b stable/wallaby https://github.com/openstack/neutron.git
sudo cp neutron/tools/ovn_migration/tripleo_environment/ovn_migration.sh /usr/bin/
sudo chmod +x /usr/bin/ovn_migration.sh
cp -r neutron/tools/ovn_migration/tripleo_environment/playbooks/ ~/ovn_migration

b) Comment the line that calls check_stack function (see below)
case $command in
    generate-inventory)
        # check_stack
        oc_check_public_network
        generate_ansible_inventory_file
        generate_ansible_config_file
        ret_val=$?
        ;;




3. Follow the official documentation until step 9 - try to generate inventory 
/home/stack/ovn_migration/start-ovn-migration.sh generate-inventory > /home/stack/ovn_migration/generate-inventory.log 
 

Actual results:
inventory file generated but is not valid, [ovn-dbs], [ovn-controllers], [dhcp] sections are empty, see [1] below

Expected results:
A valid inventory file is generated, see example below [2]

Additional info:

[1]

 [stack@undercloud-0 ovn_migration]$ cat hosts_for_migration
[ovn-dbs]

[ovn-controllers]

[dhcp]


[overcloud-controllers:children]
dhcp

[overcloud:children]
ovn-controllers
ovn-dbs


[overcloud:vars]
remote_user=heat-admin
public_network_name=public
image_name=cirros-0.5.2-x86_64-disk.img
working_dir=/home/stack/ovn_migration
server_user_name=cirros
validate_migration=True
overcloud_ovn_deploy_script=/home/stack/overcloud-deploy-ovn.sh
overcloudrc=/home/stack/overcloudrc
ovn_migration_backups=/var/lib/ovn-migration-backup

[overcloud-controllers:vars]
remote_user=heat-admin
public_network_name=public
image_name=cirros-0.5.2-x86_64-disk.img
working_dir=/home/stack/ovn_migration
server_user_name=cirros
validate_migration=True
overcloud_ovn_deploy_script=/home/stack/overcloud-deploy-ovn.sh
overcloudrc=/home/stack/overcloudrc
ovn_migration_backups=/var/lib/ovn-migration-backup





[2]


[ovn-dbs]
controller-0 ansible_host=192.168.24.29 ovn_central=true ansible_ssh_user=heat-admin ansible_become=true
controller-1 ansible_host=192.168.24.50 ansible_ssh_user=heat-admin ansible_become=true
controller-2 ansible_host=192.168.24.20 ansible_ssh_user=heat-admin ansible_become=true

[ovn-controllers]
compute-0 ansible_host=192.168.24.46 ansible_ssh_user=heat-admin ansible_become=true ovn_controller=true
compute-1 ansible_host=192.168.24.12 ansible_ssh_user=heat-admin ansible_become=true ovn_controller=true
controller-0 ansible_host=192.168.24.29 ansible_ssh_user=heat-admin ansible_become=true ovn_controller=true
controller-1 ansible_host=192.168.24.50 ansible_ssh_user=heat-admin ansible_become=true ovn_controller=true
controller-2 ansible_host=192.168.24.20 ansible_ssh_user=heat-admin ansible_become=true ovn_controller=true

[dhcp]
controller-0 ansible_host=192.168.24.29 ansible_ssh_user=heat-admin ansible_become=true
controller-1 ansible_host=192.168.24.50 ansible_ssh_user=heat-admin ansible_become=true
controller-2 ansible_host=192.168.24.20 ansible_ssh_user=heat-admin ansible_become=true


[overcloud-controllers:children]
dhcp

[overcloud:children]
ovn-controllers
ovn-dbs


[overcloud:vars]
remote_user=heat-admin
public_network_name=nova
image_name=cirros-0.5.2-x86_64-disk.img
working_dir=/home/stack/ovn_migration
server_user_name=cirros
validate_migration=True
overcloud_ovn_deploy_script=/home/stack/overcloud-deploy-ovn.sh
overcloudrc=/home/stack/overcloudrc
ovn_migration_backups=/var/lib/ovn-migration-backup

[overcloud-controllers:vars]
remote_user=heat-admin
public_network_name=nova
image_name=cirros-0.5.2-x86_64-disk.img
working_dir=/home/stack/ovn_migration
server_user_name=cirros
validate_migration=True
overcloud_ovn_deploy_script=/home/stack/overcloud-deploy-ovn.sh
overcloudrc=/home/stack/overcloudrc
ovn_migration_backups=/var/lib/ovn-migration-backup