Currently tox -epep8 will run flake8 on whole code.
To make this fast, flake8 support is added for only
updated(delta) code.
Same can be run by "tox -efast8".
Change-Id: I9c55fed32ae3060c21ec278398e9e07fb4a0fe13
Implements: BP tox-pep8-diff-part-only