Autoyast: do not overwrite firewall settings

@mchf, @schubi2: A fix for https://bugzilla.novell.com/show_bug.cgi?id=897129
This retargets #260 to the SLE12 branch.
I personally dislike this change (adding param). It makes the API more complex and it seems to be useful for AY only. 

So, how is this method used in AY? Wouldn't be appending ```unless Mode.autoinstall``` at line bellow enough?