code: def config_keys(self, sortkey = False):
          
          if sortkey:
              items = sorted(self.items())
          else:
              items = self.items()
          for k,v in items:
              if isinstance(v, ConfigTree):
                  for k2 in v.config_keys(sortkey):
                      yield k + '.' + k2
              else:
                  yield k
docs: Return all configuration keys in this node, including configurations on children nodes.