adding keras_preprocessing dependency for DGA detection

[dglauche]

keras_preprocessing is needed for the prebuilt DGA detection

[pdrieger]

Hi @dglauche thanks for flagging this. Since version 5.1.1 we tried to move all ESCU related content into a dedicated image. I also think we forced a specific tensorflow version to avoid this issue with keras_preprocessing which was reported a few times. Did you try to run the DGA detection with the ESCU image ? This would be the preferred way. Happy to explore to add keras_preprocessing to the golden image, but I would avoid it for the minimal one as this was designed to give a minimal starting point to derive others. Kindly let me know your thoughts. Many thanks in advance!