[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.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/