Our docker python-base image is a python3 image where python and python3
point to the same version of python which happens to be a 3.x depending
on the specific base image.
One of the things that came up at the PTG is that we'd like to start
identifying where we use python2 in order to port to python3. That is
made a bit more difficult when we use `python` assuming it is `python3`
as we do in our docker images.
Update the images to use `python3` to make this a lot more explicit and
clear that we don't need to port these items.
Change-Id: I54e8f128f0cb8fbbdf54e3748384470262bef3a9
421a71e145