Return the placement group, create it if it doesn't yet exist. 
    (needed for cluster type instances).

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