libjuju get_status appears to return None occassionally for the list of units for an application

[ajkavanagh]

Evidence:

13:38:23 2021-01-06 13:38:14 [WARNING] get_provider_type from zaza.openstack.utilities.juju is deprecated. Please use the equivalent from zaza.utilities.juju
13:38:23 2021-01-06 13:38:14 [INFO] Using config /var/lib/jenkins/.local/share/juju/clouds.yaml
13:38:23 2021-01-06 13:38:14 [INFO] AUTH_URL: http://10.245.161.156:5000/v3, api_ver: 3
13:38:23 2021-01-06 13:38:14 [INFO] Using keystone API V3 (or later) for undercloud auth
13:38:23 2021-01-06 13:38:16 [WARNING] unknown delta type: id
13:38:23 2021-01-06 13:38:20 [WARNING] unknown delta type: id
13:38:23 /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 of cryptography. Please upgrade your Python.
13:38:23   from cryptography.hazmat.backends import default_backend
13:38:23 Traceback (most recent call last):
13:38:23   File "/tmp/tmp.KrWZIk35cT/mojo-openstack-specs/trusty/osci-mojo/spec/specs/full_stack/stable_to_next_ha/mitaka/network_setup.py", line 20, in <module>
13:38:23     cacert=cacert))
13:38:23   File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/zaza/openstack/configure/network.py", line 303, in run_from_cli
13:38:23     options, 'no_use_juju_wait'))
13:38:23   File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/zaza/openstack/configure/network.py", line 221, in setup_gateway_ext_port
13:38:23     current_release = openstack_utils.get_os_release()
13:38:23   File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/zaza/openstack/utilities/openstack.py", line 1620, in get_os_release
13:38:23     release_pair = get_current_os_release_pair(application=application)
13:38:23   File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/zaza/openstack/utilities/openstack.py", line 1593, in get_current_os_release_pair
13:38:23     machine = list(juju_utils.get_machines_for_application(application))[0]
13:38:23   File "/var/lib/jenkins/tools/0/charm-test-infra/.tox/clients/lib/python3.5/site-packages/zaza/utilities/juju.py", line 144, in get_machines_for_application
13:38:23     for unit in status.get("units").keys():
13:38:23 AttributeError: 'NoneType' object has no attribute 'keys'

e.g. paraphrasing to the used code-path:

status = get_full_juju_status(model_name=model_name)
status = status.applications.get(application)
for unit in status.get("units").keys():
#                          ^^^^^ here status is NoneType
    yield status.get("units").get(unit).get("machine")

Proposed workaround: try again a few times until it returns something or then die.