Quads and hexes don't work with shared vertex ghost mode

[Jan Blechta]

There is missing some piece of functionality so that generation of quads/hexes fails with shared_vertex, see https://bitbucket.org/fenics-project/dolfin/src/2570a75ba20ce79b6fd5142e000dc0d4b0148465/test/unit/python/mesh/test_mesh.py?at=master&fileviewer=file-view-default#test_mesh.py-492.