This test update is modelled after the devstack base job which sets
devstack_local_conf variables with many nested layers of dicts which
child jobs never override.
In Job._deepUpdate, only coerce to dict if needed to perform the update.
Create a new json serializer helper which can handle serializing
mappingproxy types, and let that handle all of the coercion upon
Co-Authored-By: James E. Blair <firstname.lastname@example.org>