Trigger an upgrade task.

def trigger(self, identifier, force=True):
        """Trigger an upgrade task."""
        self.debug(identifier)
        url = "{base}/{identifier}".format(
            base=self.local_base_url,
            identifier=identifier
        )
        param = {}
        if force:
            param['force'] = force
        encode = urllib.urlencode(param)
        if encode:
            url += "?"
            url += encode
        return self.core.update(url, {})