make tutorial directory a variable (and don't run jupyter as root)

[ofenloch]

Instead of the hardcoded "/tf" a variable ${TENSORFLOW_NOTEBOOK_DIR} should be used.

When I derived my own image from one of yours I let jupyter notebook run as non-root user and could not access the tensorflow-tutorials notebooks in /tf/tensorflow-tutorials.

In the long run, jupyter should not run as root - and users shouldn't have to adjust this on their own.