Series upgrade tests sometimes fail on initializing flavors

Happened here: http://osci:8080/job/mojo_runner/23837/console

01:37:59 2020-09-26 01:37:56 [INFO] Initializing flavors if necessary
01:37:59 /var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/paramiko/transport.py:33: CryptographyDeprecationWarning: Python 3.5 support will be dropped in the next release ofcryptography. Please upgrade your Python.
01:37:59 from cryptography.hazmat.backends import default_backend
01:37:59 Traceback (most recent call last):
01:37:59 File "/tmp/tmp.ZXGuZPUZco/mojo-openstack-specs/xenial/osci-mojo/spec/specs/full_stack/next_series_upgrade/queens/simple_os_checks.py", line 131, in <module>
01:37:59 sys.exit(main(sys.argv))
01:37:59 File "/tmp/tmp.ZXGuZPUZco/mojo-openstack-specs/xenial/osci-mojo/spec/specs/full_stack/next_series_upgrade/queens/simple_os_checks.py", line 89, in main
01:37:59 init_flavors(novac)
01:37:59 File "/tmp/tmp.ZXGuZPUZco/mojo-openstack-specs/xenial/osci-mojo/spec/specs/full_stack/next_series_upgrade/queens/simple_os_checks.py", line 41, in init_flavors
01:37:59 names = [flavor.name for flavor in nova_client.flavors.list()]
01:37:59 File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/novaclient/v2/flavors.py", line 148, in list
01:37:59 return self._list("/flavors%s" % detail, "flavors", filters=qparams)
01:37:59 File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/novaclient/base.py", line 254, in _list
01:37:59 resp, body = self.api.client.get(url)
01:37:59 File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/keystoneauth1/adapter.py", line 395, in get
01:37:59 return self.request(url, 'GET', **kwargs)
01:37:59 File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/novaclient/client.py", line 78, in request
01:37:59 raise exceptions.from_response(resp, body, url, method)
01:37:59 novaclient.exceptions.ClientException: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.

I suggest to decorate this line [0] with some tenacity/retry as we did here [1].

0: https://github.com/openstack-charmers/openstack-mojo-specs/blob/master/helper/tests/simple_os_checks.py#L41
1: https://github.com/openstack-charmers/zaza-openstack-tests/pull/430/files