Fix oauth_whitelists in BaseSecurityManager

[jedcunningham]

Recent typing changes in BaseSecurityManager broke oauth integrations. This fixes it by initializing oauth_whitelists as an empty dict, like it is in FAB.

Without a whitelist:

  File "/Users/jedc/Envs/af/lib/python3.7/site-packages/flask_appbuilder/security/views.py", line 674, in oauth_authorized
    if provider in self.appbuilder.sm.oauth_whitelists:
AttributeError: 'AirflowSecurityManager' object has no attribute 'oauth_whitelists'

With a whitelist:

  File "/Users/jedc/github/airflow/airflow/www/fab_security/manager.py", line 238, in __init__               
    self.oauth_whitelists[provider_name] = _provider["whitelist"]                              
AttributeError: 'AirflowSecurityManager' object has no attribute 'oauth_whitelists'