python: Add flak8 env
    
    Flake8 provides basic linting and style checks for python code, helping
    to keep the source in good shape.
    
    Running tox will run flake8 automatically, showing errors in the source
    and statistics for current issues. To run only flake8 tests use:
    
        $ tox -e flake8
    
    Running tox will fail now with these errors:
    
    1     E203 whitespace before ','
    1     E265 block comment should start with '# '
    7     E302 expected 2 blank lines, found 1
    1     E305 expected 2 blank lines after class or function definition, found 1
    14    E501 line too long (82 > 79 characters)
    1     F401 'stat' imported but unused
    5     F403 'from .constants import *' used; unable to detect undefined names
    44    F405 'MiB' may be undefined, or defined from star imports: .constants, .units
    1     W391 blank line at end of file
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>