Use default `PLATFORM_NAME` in AWS settings

Rather than require the `PLATFORM_NAME` to be specified in the
`ENV_TOKENS`, allow the default setting in `common.py` to be used.

lms/envs/aws.py

@@ -99,7 +99,7 @@ CELERY_QUEUES = {
 with open(ENV_ROOT / CONFIG_PREFIX + "env.json") as env_file:
     ENV_TOKENS = json.load(env_file)
 
-PLATFORM_NAME = ENV_TOKENS['PLATFORM_NAME']
+PLATFORM_NAME = ENV_TOKENS.get('PLATFORM_NAME', PLATFORM_NAME)
 
 SITE_NAME = ENV_TOKENS['SITE_NAME']
 SESSION_COOKIE_DOMAIN = ENV_TOKENS.get('SESSION_COOKIE_DOMAIN')