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.

deploy/aws/atst-worker-envvars-configmap.yml

@@ -7,4 +7,4 @@ metadata:
 data:
   TZ: UTC
   DISABLE_CRL_CHECK: "True"
-  CRL_STORAGE_PROVIDER: CLOUDFILES
+  SERVER_NAME: aws.atat.code.mil

deploy/azure/atst-worker-envvars-configmap.yml

@@ -7,4 +7,4 @@ metadata:
 data:
   TZ: UTC
   DISABLE_CRL_CHECK: "True"
-  CRL_STORAGE_PROVIDER: CLOUDFILES
+  SERVER_NAME: azure.atat.code.mil