ignore build directory for pep8
ignore build directory for pep8

this mirrors the excludes from nova.

Change-Id: I0ca5f02918842fc708c704fcf2845936dc1c5965

citrix-openstack-build/neutron-vpnaas,dougwig/x-neutron-vpnaas,openstack/neutron-vpnaas,openstack/neutron-vpnaas