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
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: If3c8e5e3e442257f796374edba02c3a23a85839e
Merge "Provide a quick way to run flake8"