Description of problem:
Requesting the following feature enhancement: implement ironic::rpc_thread_pool_size for undercloud tuning

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
~~~


ironic::config is not included by instack-undercloud and I created BZ1631009 to include this. However, perhaps we can add the same parameter to the ironic class so that e.g. ironic::rpc_thread_pool_size: <x>  works?