include tensorflow with selectable cpu or gpu versions

[pseudotensor]

https://www.tensorflow.org/install/install_linux http://www.python36.com/how-to-install-tensorflow-gpu-with-cuda-9-2-for-python-on-ubuntu/

Power, e.g.: https://developer.ibm.com/code/howtos/install-tensorflow-on-power

Point is power needs to have tf compiled, and currently tf 1.8-1.9 only pip installs for cuda9.0. If need to support other archs/installs, need to include tf ourselves.

Use h2oai dynamic selection of cpu or gpu backends so works on cpu only systems.