Add 'optional' to the list of known label attributes

Fixes: "Unknown label: optional"

changelog/gerrit.py

@@ -96,7 +96,7 @@ def __init__(self, url, obj):
         for lbl in obj['labels']:
             self.labels[lbl] = {}
             for k, v in obj['labels'][lbl].items():
-                if k in ('blocking', 'value', 'default_value', 'description', 'values', 'all'):
+                if k in ('blocking', 'value', 'default_value', 'description', 'optional', 'values', 'all'):
                     self.labels[lbl][k] = v
                 elif k in ('approved', 'rejected', 'recommended', 'disliked'):
                     self.labels[lbl][k] = GerritUser(url, v)