Fix issue with missing setting in oauth SyncRepo task (#3656)

This was looking for setting DEFAULT_PRIVACY_LEVEL, which wasn't set by default
in our settings.

Author: agjohnson

readthedocs/settings/base.py

@@ -290,6 +290,7 @@ def USE_PROMOS(self):  # noqa
     # RTD Settings
     REPO_LOCK_SECONDS = 30
     ALLOW_PRIVATE_REPOS = False
+    DEFAULT_PRIVACY_LEVEL = 'public'
     GROK_API_HOST = 'https://api.grokthedocs.com'
     SERVE_DOCS = ['public']