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.
"In this phase all settings should be configured by installer's 1st stage correctly." Hm, that's not completely right. Firewall settings will be written in the second stage only. So the settings has to be taken from the autoinst.xml file. In the case of "keep installed network", the network settings has to be read from system (which have been written to system in first stage) BUT the firewall settings has to be taken from autoinst.xml file. Yes I know, that's a real special case but it is a real use case.
But maybe I am wrong here or the workflow of the modules is not "optimal". You and Martin are the network experts :-)