code: def get_cluster_placement_group(self):
      
      placement_group_name = 'pg-%s' % (self._name)
      try:
        self.ec2.create_placement_group(placement_group_name, 
                                                   strategy = 'cluster')
      except:
        pass
      return placement_group_name
docs: Return the placement group, create it if it doesn't yet exist. 
      (needed for cluster type instances).