This allows to ignore directories, e.g.
ignore in compute api the incubation and v1 dirs with:
openstack-doc-test --check-build --api-site \
--ignore-dir=incubation --ignore-dir=openstack-compute-api-1.0
Change-Id: I6bad41543ed2269217a2ed36fcd947b83707daf2
f0c6cd4871