This PR adds trailing unit tests to check the service cluster IP range and
improves the code coverage of k8s.io/kubernetes/cmd/kube-apiserver/app from
5.7% to 6.2%.
1) Dual stack IPv4/IPv6
2) Invalid IPv4, IPv6 mask
3) missing IPv4, IPv6 mask
4) invalid IP address format
The tests 2, 3, 4 are suggsted by Antonio Ojea.