Fix infra docs building
Fix infra docs building

The infra docs post jobs fail now since sphinx_doc_build is undefined,
rename to sphinx_build_dir - following change
I7c3d578a00a4aa4c9e175f5f83e3171c6f6a4a61.

Change-Id: Ib8c922f68a5b5f43d5b17e702020f0c31d641508

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs