Provide default values for ExtraConfig parameters
Provide default values for ExtraConfig parameters

In Iafb291419822362be2ab6755589d49f78543dc54 we added merge
strategies for ExtraConfig. However, those would be ignored
without them in overcloud-resource-registry-puppet.j2.yaml

Depends-On: https://review.opendev.org/c/openstack/heat/+/848375
Resolves: rhbz#2102466
Change-Id: I0083538ae5655e4c1637ed802caba8f0e8dbfa94

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates