tcup default to printing documentation if run without tcup.py

There should only be one way of running tcup. To make it clear
to users that might attempt directly running the Docker image,
that they must use tcup.py, print the documentation to those
users via STDOUT. It also ensures that users get the correct
documentation for running their version of the tcup image.

Change-Id: Id18f1316c5d7ee4f64c0c2c1c6a32cfc6b648b35
Merge "tcup default to printing documentation if run without tcup.py"