Undercloud installation fails with local_mtu > 1500 in Red Hat OpenStack Platform 13

Issue

Undercloud installation fails with local_mtu > 1500 in Red Hat OpenStack Platform 13

If local_mtu in undercloud.conf is set to a value greater than 1500, the undercloud install will fail:

2018-05-22 11:24:19,743 DEBUG: An exception occurred
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2336, in install
    _post_config(instack_env, upgrade)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2002, in _post_config
    network = _ensure_neutron_network(sdk)
  File "/usr/lib/python2.7/site-packages/instack_undercloud/undercloud.py", line 2052, in _ensure_neutron_network
    provider_physical_network=PHYSICAL_NETWORK, mtu=mtu)
  File "/usr/lib/python2.7/site-packages/openstack/network/v2/_proxy.py", line 1086, in create_network
    return self._create(_network.Network, **attrs)
  File "/usr/lib/python2.7/site-packages/openstack/proxy.py", line 194, in _create
    return res.create(self)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 732, in create
    self._translate_response(response)
  File "/usr/lib/python2.7/site-packages/openstack/resource.py", line 664, in _translate_response
    exceptions.raise_from_response(response, error_message=error_message)
  File "/usr/lib/python2.7/site-packages/openstack/exceptions.py", line 205, in raise_from_response
    http_status=http_status, request_id=request_id
BadRequestException: BadRequestException: Unknown error