NGC kernel runners

[achimnol]

Let's add ai.backend.kernel subpackages for NGC images.

• TensorFlow/Caffe/etc.: it mostly works like a normal Python but without the query mode.
• PyTorch: it uses Python 3.6.7 and thus we can support the query mode. – the query mode will be supported via Jupyter kernel protocol support #5 and lablup/backend.ai-agent#93.
• DIGITS: it runs a digits server as the container entrypoint. Make it a service port.