Not able to set mtu above 1500 in undercloud.conf

Issue

• from undercloud install log

RESP BODY: {"NeutronError": {"message": "Invalid input for operation: Requested MTU is too big, maximum is 1500.", "type": "InvalidInput", "detail": ""}}
2018-07-05 10:36:30,162 DEBUG: POST call to network for https://director-test..com:13696/v2.0/networks used request id req-fc1c4287-59c8-402c-96b4-643fa074ddd1
2018-07-05 10:36:30,162 DEBUG: Manager unknown ran task network.POST.networks in 0.282201051712s
2018-07-05 10:36:30,162 INFO: Network create/update failed BadRequestException: Unknown error
2018-07-05 10:36:30,162 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
2018-07-05 10:36:30,163 ERROR:
Undercloud install failed.