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.

Copied from Matthew's work in Nova here:
  https://review.openstack.org/#/c/110746/15

Change-Id: I84f6a522da5352a703caa59975643043ec1911fe
Co-Authored-By: Matthew Gilliard <matthew.gilliard@gmail.com>
Merge "Provide a quick way to run flake8"