Follow on to / OSPR-933.
See edx-code discussion here:
https://groups.google.com/forum/#!msg/edx-code/y5I80Pp8TEk/X1NdnhkgJwAJ
with info from
There are two keys that currently need to be added (not just set) in the lms.env.json file in order to add custom fields to the registration form.
install_operations/source/configuration/customize_registration_page.rst: