Do not run tests twice
    
    The flake8 environment does not have a corresponding [testenv:flake8]
    section in tox.ini. This causes the default [testenv] section to be
    used, causing the unit tests to execute twice. The lint environment is
    responsible for running flake8.
    
    Signed-off-by: Luiz Carvalho <lucarval@redhat.com>