Provide a quick way to run flake8
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 <32ca68553ca3d14b7253f5b7760ff372b38c2c8e@gmail.com>

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