don't show YaST iscsi-client during AutoYaST installation (bnc #878061)

In fact `:auto` is used for something else. When this is a client in installation workflow and user came to that dialog by going `:back`, `:auto` means `:back`. The same applies to going forward, when going `:next`, `:auto` is handled as `:next`. See https://github.com/yast/yast-yast2/blob/0cd48d499a33291630f6ebb674dfe0be370b2893/library/control/src/modules/ProductControl.rb#L1639