'Check existence of a policy association.
:param policy_id: identity of policy that is being associated
:type policy_id: string
:param endpoint_id: identity of endpoint to associate
:type endpoint_id: string
:param service_id: identity of the service to associate
:type service_id: string
:param region_id: identity of the region to associate
:type region_id: string
:raises keystone.exception.PolicyAssociationNotFound: If there is no
match for the specified association.
:returns: None'

@abc.abstractmethod
def check_policy_association(self, policy_id, endpoint_id=None, service_id=None, region_id=None):

    raise exception.NotImplemented()
