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.

An unscientific benchmark shows that this can be quite a bit faster:

Before:
$ time ./run_tests.sh -p
Running flake8 ...
./nova/compute/manager.py:77:1: F401 'periodic_task' imported but unused

real    0m37.493s
user    0m37.391s
sys     0m0.099s

After:
$ time ./run_tests.sh -8
Running flake8 on nova/compute/manager.py
./nova/compute/manager.py:77:1: F401 'periodic_task' imported but unused

real    0m0.667s
user    0m0.624s
sys     0m0.036s

Change-Id: I8dafaab40fd0427719040a76c69a8ab05e9e4386
Merge "Provide a quick way to run flake8"