Fix reference to external_brains in TrainerController (#2237)

PR #2213 conflicted with PR #2209 on a reference to external_brains.
This change fixes the conflict.

ml-agents/mlagents/trainers/trainer_controller.py

         tf.reset_default_graph()
 
         # Prevent a single session from taking all GPU memory.
-        self.initialize_trainers(trainer_config, env.external_brains)
+        self.initialize_trainers(trainer_config, env_manager.external_brains)
         for _, t in self.trainers.items():
             self.logger.info(t)