Using --local-push-destination for openstack tripleo container image prepare command leads to undercloud failures

If you run...
openstack tripleo container image prepare default \
          --output-env-file /home/stack/containers-prepare-parameter.yaml \
          --local-push-destination

The containers-prepare-parameter.yaml has a push_destination: true in it. This leads to the following failure when you run 'openstack undercloud install'

templates/./puppet/services/haproxy-internal-tls-certmonger.yaml
jinja2 rendering normal template docker-registry.j2.yaml
rendering j2 template to file: /home/stack/tripleo-heat-installer-templates/./puppet/services/docker-registry.yaml
Exception: sequence item 0: expected string, bool found
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/tripleo_deploy.py", line 1033, in _standalone_deploy
    parsed_args)
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/tripleo_deploy.py", line 651, in _deploy_tripleo_heat_templates
    self._prepare_container_images(env, parsed_args)
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/tripleo_deploy.py", line 629, in _prepare_container_images
    env, roles_data, dry_run=True)
  File "/usr/lib/python2.7/site-packages/tripleo_common/image/kolla_builder.py", line 154, in container_images_prepare_multi
    modify_vars=modify_vars,
  File "/usr/lib/python2.7/site-packages/tripleo_common/image/kolla_builder.py", line 271, in container_images_prepare
    imagename = '/'.join((push_destination, image))
TypeError: sequence item 0: expected string, bool found
None
Install artifact is located at /home/stack/undercloud-install-20180810110859.tar.bzip2