Get ElasticSearch instance for given resource.

def elastic(self, resource=None):
        """Get ElasticSearch instance for given resource."""
        px = self._resource_prefix(resource)

        if px not in self.elastics:
            url = self._resource_config(resource, 'URL')
            assert url, 'no url for %s' % px
            self.elastics[px] = get_es(url, **self.kwargs)

        return self.elastics[px]