Provide a quick way to run flake8

"run_tests.sh -p" always checks every file for errors even though you
have probably only changed a handful. This patch adds "run_tests.sh -8"
which only checks the files that were modified in the HEAD commit or
in the current working tree.

It is borrowed from the recent nova commit b011325cc6.
The related ML post is:
http://lists.openstack.org/pipermail/openstack-dev/2014-August/043346.html

Change-Id: Ieeabf1b79a0790af8bde3144dca59b4245a0b79a
Merge "Provide a quick way to run flake8"