Update Dockerfile ENTRYPOINT to use /usr/local/bin/python (kubeflow#744)

Base image `FROM tensorflow/tensorflow:1.15.2-py3` uses python3 and therefore the python binary location is `/usr/bin/python3`. However, [tensorflow base image creates a symlink](https://github.com/tensorflow/tensorflow/blob/e5bf8de410005de06a7ff5393fafdf832ef1d4ad/tensorflow/tools/dockerfiles/dockerfiles/cpu.Dockerfile#L45) to the current python binary as `/usr/local/bin/python` regardless if that is python version 2 or version 3, so that binary location should be used in the *ENTRYPOINT* of the `Dockerfile.model` instead of `/usr/bin/python` which is customary for Python v2.x installations.

mnist/Dockerfile.model

@@ -5,5 +5,5 @@ FROM tensorflow/tensorflow:1.15.2-py3
 ADD model.py /opt/model.py
 RUN chmod +x /opt/model.py
 
-ENTRYPOINT ["/usr/bin/python"]
+ENTRYPOINT ["/usr/local/bin/python"]
 CMD ["/opt/model.py"]