Handle interfaces with multiples IPv4 addresses

After a vagrant up of https://github.com/kelseyhightower/kubernetes-coreos my master and minions nodes have an interface with two IPv4 address:
$ ip addr show
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:f6:f6:a3 brd ff:ff:ff:ff:ff:ff
    inet 192.168.56.102/24 brd 192.168.56.255 scope global dynamic enp0s8
       valid_lft 1024sec preferred_lft 1024sec
    inet 172.17.8.100/24 brd 172.17.8.255 scope global enp0s8
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fef6:f6a3/64 scope link 
       valid_lft forever preferred_lft forever

$ journalctl -u rudder
Sep 16 09:36:04 minion-1 systemd[1]: Starting rudder.service...
Sep 16 09:36:04 minion-1 systemd[1]: Started rudder.service.
Sep 16 09:36:04 minion-1 rudder[957]: E0916 09:36:04.645052 00957 main.go:75] Error looking up interface 172.17.8.101: No interface with given IP found


No need to check again for equality here -- GetIfaceIP4AddrMatch already asserted that. However, with the proposal above, this should just check the error and boolean return code.