The Celery worker cannot render URLs for the app without having a
SERVER_NAME value set. AT-AT's ability to send notifications when an
environment is ready is broken as a result.
This commit sets a null default value for SERVER_NAME in the default
config file. A setting must exist in the INI file in order to be
over-written by an environment variable, which is why we declare it as
null here. There is an additional kwarg, "allow_no_value", that must be
passed to ConfigParser to allow null values.
This also applies the correct domains as SERVER_NAME environment
variables in the Kubernetes ConfigMaps for the AWS and Azure Celery
workers.
fc637e933d