Add a copyright notice to network.go
Add a copyright notice to network.go

The notice was copied from other files in the commit
which introduced this file. [1]
Except that I bumped the year to 2018.

Background: I have a copy of this file in my project, [2]
which uses check-copyrights.sh style checking to ensure
every files have a copyright notice.

[1] 218378bc7e484fe70c72d23dd6c9f1a9a612ec64
[2] https://github.com/midonet/midonet-kubernetes

projectcalico/calico-cni,projectcalico/calico-rkt,projectcalico/calico-cni,projectcalico/calico-cni