Fix pylint error in ipapython/dn.py
    
    ipapython/dn.py:1324: [R1710(inconsistent-return-statements), DN.__contains__]
    Either all return statements in a function should return an expression, or none of them should.)
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>