Add feature flag to disable username confirmation

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

util/config/schema.py

@@ -909,6 +909,13 @@ CONFIG_SCHEMA = {
       'description': 'If set to true, users can rename their own namespace. Defaults to False',
       'x-example': True,
     },
+
+    # Feature Flag: Username confirmation.
+    'FEATURE_USERNAME_CONFIRMATION': {
+      'type': 'boolean',
+      'description': 'If set to true, users can confirm their generated usernames. Defaults to True',
+      'x-example': False,
+    },
  },
 }