Fix ./run_tests.sh --pep8

Create a common script to call flake8 to share flake8 options and
make tox and run_tests.sh use this script.

Fixes bug: 1179331

Change-Id: I92dc54fa3f4bb22aa39bde226b6d769088f9a2e3
Merge "Fix ./run_tests.sh --pep8"