suspend support for sphinxcontrib.images

the sphinxcontrib.image extension was broken with sphinx 16.x.
however, sphinx 15.x breaks with newer version of the multiprocessor module.
so we suspend the thumbnail processing and lift the lock to sphinx 15.x

also, the number of parallel sphinx tasks is can be overridden with SPHINXEXTRA="-j #'.
default is to try use all local CPU cores.