[schema2sql] properly consider skip_relations parameters.
This is the same bug as yams #286912. Also, introducing a unique function
telling if the relation should have a table or not unhide some other problems
related to computed relation, or missing skipped relations information.
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc