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.
Merge pull request #474 from mtomaschewski/systemd-ordering-1

systemd: wicked ordering dependency fixes (bsc#895600,bsc#901337)
Looks good, by code inspection.