Azure: Allow distros to install Python they want
    
    The platforms may have different Pythons.
    But due to [0] the Python installed via the 'UsePythonVersion@0'
    task should be compatible with the container's 'libpythonxx.so'.
    'AZURE_PYTHON_VERSION' platform variable is introduced to cover
    this. So, if your distro has Python3.8, set the mentioned variable
    to '3.8', later, this version will be installed by the
    'UsePythonVersion@0' Azure task for 'WebUI_Unit_Tests' and 'Tox'
    jobs.
    
    To allow tox to run any Python3 environment the 'py3' one is used.
    'py3' is the well-known Tox's environment, which utilizes 'python3'
    executable.
    
    [0]: https://github.com/microsoft/azure-pipelines-tasks/issues/11070
    
    Fixes: https://pagure.io/freeipa/issue/8254
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>