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.
Unfortunately it is not enough because this happens only if "keep_install_network" is set. While calling Lan.Read the Lan.rb module does not know anything about that flag because Lan.Import will be called after it.