Run pep8 check in run_tests.sh as in tox

Until now the flake8 tests run by tox and run_tests.sh were a bit
different - run_tests.sh was not checking heat-* python scripts in bin
folder and was checking the docs folder that is ignored in tox' pep8
check.

This patch brings pep8 style checking done by run_tests.sh in line with
what is done by tox (and thus the gate).

Change-Id: Ie3f2c25210075df6b941854737f0d926d141601b
Merge "Run pep8 check in run_tests.sh as in tox"