build containers: Allow setting of Python version used.

Currently the pipeline used to create test containers is using Python
3.6.15, and Ansible 2.12 requires, at least, Python 3.8.

This change adds a new parameter to build container template,
`python_version`, which is set by default to '3.x', meaning it will use
the latest Python version available (for version 3) if the parameter is
not explicitly set.