Add missing pre_common_callback to stageuser_add
    
    This pre_callback contains method to preprocessing usercertificate that
    was not called during stageuser-add. This commit adds missing
    pre_common_callback call to stageuser_add.
    
    https://fedorahosted.org/freeipa/ticket/5759
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>