Undercloud upgrade from Red Hat OpenStack Platform 11 to 12 yields "No conductor service registered which supports driver"

Issue

At the end of an undercloud upgrade from Red Hat OpenStack Platform 11 to 12, one gets the following error message:

  File "/usr/lib/python2.7/site-packages/ironicclient/common/base.py", line 191, in _update
    resp, body = self.api.json_request(method, url, body=patch)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 558, in json_request
    resp = self._http_request(url, method, **kwargs)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 188, in wrapper
    return func(self, url, method, **kwargs)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 540, in _http_request
    error_json.get('debuginfo'), method, url)
BadRequest: No valid host was found. Reason: No conductor service registered which supports driver pxe_ipmitool. (HTTP 400)
2018-08-15 21:40:40,059 ERROR: 
#############################################################################
Undercloud upgrade failed.

Reason: No valid host was found. Reason: No conductor service registered which supports driver pxe_ipmitool. (HTTP 400)

See the previous output for details about what went wrong.  The full install
log can be found at /home/stack/.instack/install-undercloud.log.

#############################################################################

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 1875, in install
    _post_config(instack_env)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 1771, in _post_config
    _ensure_node_resource_classes(ironic)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 1501, in _ensure_node_resource_classes
    'value': DEFAULT_NODE_RESOURCE_CLASS}])
  File "/usr/lib/python2.7/site-packages/ironicclient/v1/node.py", line 336, in update
    method=http_method)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/base.py", line 191, in _update
    resp, body = self.api.json_request(method, url, body=patch)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 558, in json_request
    resp = self._http_request(url, method, **kwargs)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 188, in wrapper
    return func(self, url, method, **kwargs)
  File "/usr/lib/python2.7/site-packages/ironicclient/common/http.py", line 540, in _http_request
    error_json.get('debuginfo'), method, url)
ironicclient.common.apiclient.exceptions.BadRequest: No valid host was found. Reason: No conductor service registered which supports driver pxe_ipmitool. (HTTP 400)
Command 'instack-upgrade-undercloud' returned non-zero exit status 1
[stack@undercloud ~]$

How can this be fixed?