User Tracker: creation of user with minimal values
    
    Fix provide possibility to create user-add test with minimal values,
    where uid is not specified, to provide better coverage. Also provide
    check for non-empty unicode string for attributes required in init method
    
    https://fedorahosted.org/freeipa/ticket/6126
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Milan Kubik <mkubik@redhat.com>
    Reviewed-By: Lenka Doudova <ldoudova@redhat.com>