Add feature flag to disable username confirmation

Fixes https://jira.coreos.com/browse/QUAY-914

endpoints/api/user.py

@@ -392,7 +392,8 @@ class User(ApiResource):
         new_username = user_data.get('username')
         previous_username = user.username
 
-        rename_allowed = features.USER_RENAME or confirm_username
+        rename_allowed = (features.USER_RENAME or 
+                          (confirm_username and features.USERNAME_CONFIRMATION))
         username_changing = new_username and new_username != previous_username
 
         if rename_allowed and username_changing:

endpoints/oauth/login.py

@@ -110,6 +110,7 @@ def _conduct_oauth_login(auth_system, login_service, lid, lusername, lemail, met
     user_obj = model.user.create_federated_user(new_username, lemail, service_id, lid,
                                                 set_password_notification=requires_password,
                                                 metadata=metadata or {},
+                                                confirm_username=features.USERNAME_CONFIRMATION,
                                                 prompts=prompts,
                                                 email_required=features.MAILING)