Provide a quick way to run flake8

Patch copied from nova:
https://review.openstack.org/#/c/110746/

'run_tests.sh -p' always checks every file for errors.
This patch adds 'run_tests.sh -8' which only checks the files
that were modified in HEAD commit or the current working tree.

time ./run_tests.sh -p
Running flake8 ...
./glance/async/eventlet_executor.py:29:80: E501 line too
long (82 > 79 characters)

real    0m11.489s
user    0m11.342s
sys     0m0.136s

time ./run_tests.sh -8
Running flake8 on doc/source/configuring.rst
etc/glance-api.conf glance/async/eventlet_executor.py run_tests.sh
./glance/async/eventlet_executor.py:29:80: E501 line too
long (82 > 79 characters)

real    0m0.604s
user    0m0.549s
sys     0m0.059s

Change-Id: I5fa9e85da017da3989a76caf95831f3ac927d1b7
Merge "Provide a quick way to run flake8"