config_template broken on ansible 2.3?

The openstack_user_config_overrides defined in https://gist.githubusercontent.com/Logan2211/61395dfd05af1819673bb5232d14077f/raw/d33878dc906913a9fba19635a2787556fa5f2422/bootstrap_host_overrides.yml and being consumed here https://github.com/openstack/openstack-ansible/blob/b55fc7240d31732d38119c421b479cd3f426e669/tests/roles/bootstrap-host/tasks/prepare_aio_config.yml#L38-L46 is not working on either python2.7 or python3 in master w/ ansible 2.3.

With ansible verbosity increased, the output looks like:
TASK [bootstrap-host : Deploy openstack_user_config] ***************************
task path: /home/ubuntu/workspace/openstack-upstream/openstack-ansible/tests/roles/bootstrap-host/tasks/prepare_aio_config.yml:38
<localhost> ESTABLISH LOCAL CONNECTION FOR USER: root
<localhost> EXEC /bin/sh -c 'echo ~ && sleep 0'
<localhost> EXEC /bin/sh -c '( umask 77 && mkdir -p "` echo /root/.ansible/tmp/ansible-tmp-1501255690.69-280917705831294 `" && echo ansible-tmp-1501255690.69-280917705831294="` echo /root/.ansible/tmp/ansible-tmp-1501255690.69-280917705831294 `" ) && sleep 0'
The full traceback is:
Traceback (most recent call last):
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 125, in run
    res = self._execute()
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/ansible/executor/task_executor.py", line 522, in _execute
    result = self._handler.run(task_vars=variables)
  File "/etc/ansible/roles/plugins/action/_v2_config_template.py", line 598, in run
    ignore_none_type=_vars.get('ignore_none_type', True)
  File "/etc/ansible/roles/plugins/action/_v2_config_template.py", line 407, in return_config_overrides_yaml
    width=1000,
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/__init__.py", line 202, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/__init__.py", line 190, in dump_all
    dumper.represent(data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 28, in represent
    node = self.represent_data(data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 225, in represent_dict
    return self.represent_mapping(u'tag:yaml.org,2002:map', data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 225, in represent_dict
    return self.represent_mapping(u'tag:yaml.org,2002:map', data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 67, in represent_data
    node = self.yaml_representers[None](self, data)
  File "/opt/ansible-runtime/local/lib/python2.7/site-packages/yaml/representer.py", line 249, in represent_undefined
    raise RepresenterError("cannot represent an object: %s" % data)
RepresenterError: cannot represent an object: 172.29.236.0/22