Fixed network_autoconfiguration_test stubbing on s390 (bnc#883836).

This particular test does not want to test s390 specialties so make it
see a boring machine.

This build will show whether the fix has worked: https://build.suse.de/package/live_build_log/home:mvidner:branches:SUSE:SLE-12:GA/yast2-network/standard/s390x