Add build dir to flake8 exclude list
Add build dir to flake8 exclude list

The `build` directory is generated from command `python setup.py install`,
we need to add that to flake8 exclude list in case pep8 test fails.

Change-Id: I3906ac973a9c1f73d1917c0bd0bc3bd81199a31f

openstack/barbican,openstack/barbican