Feat: Set python3 version from pyenv

Use pyenv whih is the standard way to manage, set and use a python3
installation on the system.

The required version of python3 for all jobs should be > 3.8.x, to
avoid PyPI dependencies conflicts with outdated versions. However the
lf-activate-venv() uses the system default version python installed
through packages. This can cause warning and build failures that source
lf-env.sh.

Update lf-activate-venv to use pyenv versions of python3 installed
through the lfit.python-install galaxy ansible role.

Issue-ID: RELENG-4357
Change-Id: I821a86ac3b54f284e853316f0d782ad551249925
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>