Allow jobs to set python3 version using alternatives on the builder node. The builder should be able check and/or sets the version of Python 3.x on the executor node.
 

This is required since some of the jobs are picking up 3.6 although 3.8 is installed and available on the builder nodes.

```

12:48:39 PYTHON=python3
12:48:39 
12:48:39 [EnvInject] - Variables injected successfully.
12:48:39 [lf-infra-global-jjb-rtd-verify-any] $ /bin/bash -l /tmp/jenkins103571275582208542.sh
12:48:39 ---> tox-install.sh
12:48:39 + python3 -m pip install --user --quiet --upgrade tox tox-pyenv virtualenv
12:48:43 + python3 --version
12:48:43 Python 3.6.8
12:48:43 + python3 -m pip --version
12:48:43 pip 21.3.1 from /home/jenkins/.local/lib/python3.6/site-packages/pip (python 3.6)
12:48:43 + python3 -m pip freeze

```