Add more dir exceptions to pep8 tox job
Add more dir exceptions to pep8 tox job

Manila has more dynamically created dirs than we exclude in
pep8 job. Such as 'build' and 'cover'.
Therefore, exclude them too to speed up passing of pep8 job on dev
machine where lots of dynamically created things exist.

TrivialFix

Change-Id: I256720846acdd584cadaf59e9dedaad0ff741ae3

vponomaryov/manila,NetApp/manila,openstack/manila,NetApp/manila,vponomaryov/manila,bswartz/manila,openstack/manila,bswartz/manila