service: added After=network-pre in wicked (bsc#901337)

Added After=network-pre.target ordering dependency for services
which want to run before network is set up, e.g. firewall init.
service: no Before=SuSEfirewall2 in wicked (bsc#895600)

Removed unneeded Before=SuSEfirewall2.service ordering dependency.
service: removed automatically resolved targets (bsc#895600)

Removed unneeded shutdown.target and multi-user.target, which
are added by systemd automatically as needed.