Also, maybe we do want to have a default dict here after all, and also in the `AUTH_TOKENS.get()` in `aws.py`. Those dicts would indicate the default bundled providers we want to turn on, and I think a non-null example would be good.  
Actually, maybe `AUTH_DEFAULT_PROVIDERS` could just go in `common.py`, with comments noting it won't have any effect unless `FEATURES['ENABLE_THIRD_PARTY_AUTH']` is true

Something like
```
# Example dict for setting up auth providers
# This setting has no effect unless FEATURES['ENABLE_THIRD_PARTY_AUTH'] is True
# If FEATURES['ENABLE_THIRD_PARTY_AUTH'] is True, the providers corresponding to each top-level
# key are enabled, and each key-value pair in the top-level value (dict) will be patched
# into the django settings module (done by calls in aws.py and dev.py).
# For example, settings.SOCIAL_AUTH_GOOGLE_OAUTH2_KEY will be set to 'key' by the dict below.
# To use in the dev environment, you should override the settings to provide real values in private.py,
# e.g. SOCIAL_AUTH_GOOGLE_OAUTH2_KEY = "<real_key>"
# To use in Vagrant and in Prod, the json form of this dict, containing real values,
# should be provided in lms.auth.json.
AUTH_DEFAULT_PROVIDERS = {
    'Google': {
        'SOCIAL_AUTH_GOOGLE_OAUTH2_KEY': 'key',
        'SOCIAL_AUTH_GOOGLE_OAUTH2_SECRET': 'secret'},
    'Mozilla Persona': {}
}
```