Let us pass arguments to flake8 in tox env

Tox variable '{posargs}' contains arguments passed to tox with something
like:

 tox -- <my arguments there>

By adding '{posargs}' to the flake8 command of the flake8 env, one can
pass extra arguments to flake8, for example:

 tox -eflake8 -- --statistics --show-source

Change-Id: I7d75c282284cc7bba699fa01674efd328f0c0a25