Processing one stage.

def process(self, stage):
        """Processing one stage."""
        self.logger.info("Processing pipeline stage '%s'", self.title)
        output = []
        for entry in stage:
            key = list(entry.keys())[0]
            if key == "env":
                self.pipeline.data.env_list[1].update(entry[key])
                self.logger.debug("Updating environment at level 1 with %s",
                                  self.pipeline.data.env_list[1])
                continue

            # if not "env" then it must be "tasks" (schema):
            tasks = Tasks(self.pipeline, re.match(r"tasks\(parallel\)", key) is not None)
            result = tasks.process(entry[key])
            for line in result['output']:
                output.append(line)
            if not result['success']:
                self.event.failed()
                return {'success': False, 'output': output}

        self.event.succeeded()
        return {'success': True, 'output': output}