flake8 ignore build directory.
flake8 ignore build directory.

Change-Id: I5c1b436dac7bb558f2c2c02c0d7de51751305e91

citrix-openstack-build/python-heatclient,rdo-management/python-heatclient,tcpcloud/python-heatclient,larsks/python-heatclient,rdo-management/python-heatclient,varunarya10/python-heatclient,metacloud/python-heatclient,jasondunsmore/python-heatclient,ecerulm/python-heatclient,ecerulm/python-heatclient,metacloud/python-heatclient,larsks/python-heatclient,openstack/python-heatclient,varunarya10/python-heatclient,citrix-openstack-build/python-heatclient,tcpcloud/python-heatclient,jasondunsmore/python-heatclient