optional style for interfacing freqai with backtesting

freqtrade/optimize/backtesting.py

@@ -204,7 +204,7 @@ class Backtesting:
         """
         self.progress.init_step(BacktestState.DATALOAD, 1)
 
-        if self.config['freqai']['train_period'] > 0:
+        if self.config.get('freqai', {}).get('train_period') > 0:
             self.required_startup += int((self.config['freqai']['train_period'] * 86400) /
                                          timeframe_to_seconds(self.config['timeframe']))
             logger.info("Increasing startup_candle_count for freqai to %s", self.required_startup)