Add flake8 to tox whitelist_externals
Add flake8 to tox whitelist_externals

Stop triggering triggering a “not installed in virtualenv” every time
flake8 is called.

Change-Id: I8c242febe4e15debc5a98fde74332079456a523d

FlorentFlament/ospurge-packaging,openstack/ospurge,DigitalPacific/ospurge,tnahak/ospurge,JonasPf/ospurge,stackforge/ospurge,remoteur/ospurge,openstack/ospurge