Sync charmhelpers

hooks/charmhelpers/core/hookenv.py

@@ -214,6 +214,12 @@ class Config(dict):
         except KeyError:
             return (self._prev_dict or {})[key]
 
+    def keys(self):
+        prev_keys = []
+        if self._prev_dict is not None:
+            prev_keys = self._prev_dict.keys()
+        return list(set(prev_keys + dict.keys(self)))
+
     def load_previous(self, path=None):
         """Load previous copy of config from disk.