move flake8 args to [flake8] section of tox.ini
move flake8 args to [flake8] section of tox.ini

tox supports a `[flake8]` section, so move the default
settings there rather than having them passed in on the
command line.  Projects can still override flake8 arguments
by using `RUN_FLAKE8_EXTRA_ARGS` in `.travis/config.sh`

Signed-off-by: Till Maas <279a4cf6d7f8da1ddc113bb946b9273d9d938683@till.name>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network