How to import image data into python for keras?
The docs for ImageDataGenerator suggest that no augmentation is done by default. So you could instantiate it without any augmentation parameters and keep the rest of your code for handling your directory structure:

train_datagen = ImageDataGenerator(rescale=1./255)


You are also allowed to write your own custom data generator and pass it to model.fit_generator(). Here is a nice tutorial.

Or if your data fits in memory you could write some simpler code possibly using keras.preprocessing.image.load_img to load all the images into an array and pass them to model.fit instead.