Fallback to default ZENDESK_URL when not in JSON

This line previously departed from the convention of coalescing down to
the original value specified in `lms/envs/common.py`.

Now, the setting can fall back to its default value from `common.py`,
instead of only respecting the value loaded from JSON (or `None` if not
defined).

lms/envs/aws.py

@@ -326,7 +326,7 @@ META_UNIVERSITIES = ENV_TOKENS.get('META_UNIVERSITIES', {})
 COMMENTS_SERVICE_URL = ENV_TOKENS.get("COMMENTS_SERVICE_URL", '')
 COMMENTS_SERVICE_KEY = ENV_TOKENS.get("COMMENTS_SERVICE_KEY", '')
 CERT_QUEUE = ENV_TOKENS.get("CERT_QUEUE", 'test-pull')
-ZENDESK_URL = ENV_TOKENS.get("ZENDESK_URL")
+ZENDESK_URL = ENV_TOKENS.get('ZENDESK_URL', ZENDESK_URL)
 FEEDBACK_SUBMISSION_EMAIL = ENV_TOKENS.get("FEEDBACK_SUBMISSION_EMAIL")
 MKTG_URLS = ENV_TOKENS.get('MKTG_URLS', MKTG_URLS)