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.

atst/app.py

@@ -193,7 +193,7 @@ def make_config(direct_config=None):
     )
     OVERRIDE_CONFIG_FILENAME = os.getenv("OVERRIDE_CONFIG_FULLPATH")
 
-    config = ConfigParser()
+    config = ConfigParser(allow_no_value=True)
     config.optionxform = str
 
     config_files = [BASE_CONFIG_FILENAME, ENV_CONFIG_FILENAME]