add default param for datacontainer get function

the usage pattern looks like one should be able to do this which would come in handy

mango/agent/role.py

@@ -50,11 +50,11 @@ def __setitem__(self, key, newvalue):
     def __contains__(self, key):
         return hasattr(self, key)
 
-    def get(self, key):
+    def get(self, key, default=None):
         if key in self:
             return self[key]
         else:
-            return None
+            return default
 
     def update(self, data: dict):
         for k, v in data.items():