Description of problem:
Requesting the following feature enhancement: custom configuration for the undercloud

The undercloud allows us to push custom hierdata to make configuration
persistent:
~~~
undercloud.conf:
hieradata_override = /home/stack/custom_hiera.yaml
~~~

However, for https://access.redhat.com/articles/3597351 , I need to modify a custom ironic parameter for which no
puppet manifest exists yet (at least by grepping, I cannot find it):
~~~
ironic::config::ironic_config:
  DEFAULT/rpc_thread_pool_size:
    value: 8
~~~
However, for this to work, I need class ironic::config to be loaded.

The overcloud offers an elegant way to include custom classes, e.g.:
~~~
[stack@undercloud-7 ~]$ cat
/usr/share/openstack-tripleo-heat-templates/puppet/manifests/overcloud_controller_pacemaker.pp
# Copyright 2015 Red Hat, Inc.
# All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.

if hiera('step') >= 4 {
  hiera_include('controller_classes', [])
}

$package_manifest_name =
join(['/var/lib/tripleo/installed-packages/overcloud_controller_pacemaker',
hiera('step')])
package_manifest{$package_manifest_name: ensure => present}
~~~

And then use:
https://access.redhat.com/solutions/2328671
~~~
parameter_defaults:
  controllerExtraConfig:
    neutron::config::server_config:
      quotas/quota_port:
        value: '-1'
      quotas/quota_subnet:
        value: '-1'
      quotas/quota_network:
        value: '-1'

    controller_classes:
      - ::neutron::config
~~~

According to Alex Schulz, this is currently not supported and this would be a simple patch to
always include the various ::<class>::config methods but we need to
understand what is required for customers and why.