'Get the policy for an explicit association.
This method is not exposed as a public API, but is used by
get_policy_for_endpoint().
:param endpoint_id: identity of endpoint
:type endpoint_id: string
:param service_id: identity of the service
:type service_id: string
:param region_id: identity of the region
:type region_id: string
:raises keystone.exception.PolicyAssociationNotFound: If there is no
match for the specified association.
:returns: dict containing policy_id (value is a tuple containing only
the policy_id)'

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

    raise exception.NotImplemented()
