Undercloud or Overcloud Installation Fails with "TypeError: format() argument after ** must be a mapping, not NoneType"

Issue

• The following container-prepare-parameter.yaml file is used:

[stack@v2undercloud ~]$ cat templates/containers-prepare-parameter.yaml
# Generated with the following on 2020-01-06T11:38:02.817764
#
#   openstack tripleo container image prepare default --local-push-destination --output-env-file /home/stack/templates/containers-prepare-parameter.yaml
#
parameter_defaults:
  ContainerImagePrepare:
  - push_destination: 10.10.10.10:8787
    set:
      ceph_alertmanager_image: alertmanager
      ceph_alertmanager_namespace: docker.io/prom
      ceph_alertmanager_tag: v0.16.2
      ceph_grafana_image: grafana
      ceph_grafana_namespace: docker.io/grafana
      ceph_grafana_tag: 5.2.4
      ceph_image: rhceph-4-rhel8
      ceph_namespace: registry.redhat.io/rhceph-beta
      ceph_node_exporter_image: node-exporter
      ceph_node_exporter_namespace: docker.io/prom
      ceph_node_exporter_tag: v0.17.0
      ceph_prometheus_image: prometheus
      ceph_prometheus_namespace: docker.io/prom
      ceph_prometheus_tag: v2.7.2
      ceph_tag: 4-8
      name_prefix: openstack-
      name_suffix: ''
      namespace: registry.redhat.io/rhosp-beta
      neutron_driver: ovn
      rhel_containers: false
      tag: 16.0
    tag_from_label: '{version}-{release}'
  ContainerImageRegistryCredentials:
    registry.redhat.io:
      username: password

• When trying to prepare the images, it fails with the following:

[stack@v2undercloud ~]$ openstack tripleo container image prepare -e templates/containers-prepare-parameter.yaml
Exception occured while running the command
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tripleoclient/command.py", line 32, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/cliff/command.py", line 185, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python3.6/site-packages/tripleoclient/v1/container_image.py", line 959, in take_action
    cleanup=parsed_args.cleanup, lock=lock)
  File "/usr/lib/python3.6/site-packages/tripleo_common/image/kolla_builder.py", line 217, in container_images_prepare_multi
    lock=lock
  File "/usr/lib/python3.6/site-packages/tripleo_common/image/kolla_builder.py", line 336, in container_images_prepare
    images, tag_from_label)
  File "/usr/lib/python3.6/site-packages/tripleo_common/image/image_uploader.py", line 815, in discover_image_tags
    discover_args):
  File "/usr/lib64/python3.6/concurrent/futures/_base.py", line 586, in result_iterator
    yield fs.pop().result()
  File "/usr/lib64/python3.6/concurrent/futures/_base.py", line 425, in result
    return self.__get_result()
  File "/usr/lib64/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/lib64/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.6/site-packages/tripleo_common/image/image_uploader.py", line 2275, in discover_tag_from_inspect
    i, image, tag_from_label, fallback_tag)
  File "/usr/lib/python3.6/site-packages/tripleo_common/image/image_uploader.py", line 768, in _discover_tag_from_inspect
    tag_label = tag_from_label.format(**labels)
TypeError: format() argument after ** must be a mapping, not NoneType
format() argument after ** must be a mapping, not NoneType