CI: Fix Debian aarch64 jobs

Debian defaults to Python2 which is not complete in aarch64 images.
This patch changes CI to always use Python3.

We need to install several Python modules to have working ussuri jobs.

"Failed to import the required Python library (setuptools) on primary's Python /usr/bin/python3."

And then several Python2 ones for train->ussuri upgrade jobs:

"Unable to find any of pip2 to use. pip needs to be installed."

Change-Id: Ia0d3ff15d97d1cabbb0b8e7f32e8712ca3f94732