tox: Make it easy to run the tests
    
    tox make it easy to run python tests with multiple versions of python.
    
    To run all python tests:
    
        $ tox
    
    This command:
    - sets environment variables for the test, without modifying your shell
    - creates python virtual environment for python 2.7 and python 3.6 if
      needed.
    - installs the test dependencies in the virtual environment without
      modifying system python.
    - builds wdmd, sanlock and the python module
    - runs the tests with both python versions
    
    Note: python 3.6 fails now because the python module does not compile
    yet on python 3.
    
    To run only the python 2.7 tests:
    
        $ tox -e py27
    
    README.dev was simplified; there is not need now to install pytest,
    run make, or set environment variables.
    
    pytest.ini was merged into tox.ini.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>