nanny: enabled nanny use for hotplugging (bnc#880515)

To disable again in case of trouble, edit /etc/wicked/common.xml
and set <use-nanny>false</use-nanny>, restart wickedd.service and
execute "wicked ifup all" to start the interfaces directly.
version 0.5.37
Merge pull request #340 from mtomaschewski/event-order-4

events: process complete device state change event transitions