pybind: catch EntityAddress missing /

If the / is missing in an EntityAddress, an ArgumentValid exception must
be raised so that it can be caught in the same way other argument
validation exceptions are.

http://tracker.ceph.com/issues/6274 refs #6274

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>