Specify Flask SERVER_NAME value for Celery worker.

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.

config/base.ini

@@ -25,6 +25,7 @@ REDIS_URI = redis://localhost:6379
 RQ_QUEUES = atat_%(ENVIRONMENT)s
 SECRET = change_me_into_something_secret
 SECRET_KEY = change_me_into_something_secret
+SERVER_NAME
 SESSION_COOKIE_NAME=atat
 SESSION_TYPE = redis
 SESSION_USE_SIGNER = True