This fixes several flake8 failures and integrates flake8 into linters
tox environment.
Rule W504 is disabled because at this moment there is no known way
to avoid flapping between W504 and W503.
This allows us to retire openstack-tox-pep8 job because the more
beneric openstack-tox-linters includes it. Still, developers will
be able to coveniently run only pep8 if they want.
Change-Id: I7da0f6f09a533dd1c4dc303029e8c587bc200f66
90d9c1bc41