Description of problem:
When trying to run ovs2ovn migration on OSP17 the ovn_migration script fails on check_stack function because heat is not available and 'openstack stack show <STACK_NAME>' command does not work on OSP17 as it used to work on previous OSP versions.

(undercloud) [stack@undercloud-0 ovn_migration]$ openstack stack show overcloud
/usr/lib64/python3.6/site-packages/_yaml/__init__.py:23: DeprecationWarning: The _yaml extension module is now located at yaml._yaml and its location is subject to change.  To use the LibYAML-based parser and emitter, import from `yaml`: `from yaml import CLoader as Loader, CDumper as Dumper`.
  DeprecationWarning
public endpoint for orchestration service in regionOne region not found
(undercloud) [stack@undercloud-0 ovn_migration]$ echo $?
1



# Check if the stack exists
function check_stack {
    source $STACKRC_FILE
    openstack stack show $STACK_NAME 1> /dev/null || {
        echo "ERROR: STACK_NAME=${STACK_NAME} does not exist. Please provide the stack name or its ID "
        echo "       via STACK_NAME environment variable."
        exit 1
    }
}



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

How reproducible:
100%

Steps to Reproduce:
1. Deploy OSP17.0 with ML2/OVS backend
2. Due to BZ2035354 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

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:
ovn_migration script fails on check_stack function

Expected results:
ovn_migration script does not fail 

Additional info: