Get elastic prefix for given resource.

        Resource can specify ``elastic_prefix`` which behaves same like ``mongo_prefix``.

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

        Resource can specify ``elastic_prefix`` which behaves same like ``mongo_prefix``.
        """
        px = 'ELASTICSEARCH'
        if resource and config.DOMAIN[resource].get('elastic_prefix'):
            px = config.DOMAIN[resource].get('elastic_prefix')
        return px