OpenStack upgrade Train to Ussuri: test_500_validate_openstack_upgraded_cloud fails at pinging new instance

This test class has been [commented out for now](https://github.com/openstack-charmers/charmed-openstack-tester/blob/master/tests/openstack-upgrade/tests/tests.yaml).

To reproduce, uncomment and run

```
tox -e openstack-upgrade kitchen-sink-bionic-train
```