Flake8 should ignore build folder
Flake8 should ignore build folder

This adds 'build' to the exclude list for flake8

Fixes bug 1198329

Change-Id: Ia3688a2fc0334d32d89f04d114242d39bdc25f0e

citrix-openstack-build/keystoneauth,jamielennox/keystoneauth,sileht/keystoneauth