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


GetIfaceIP4AddrMatch either returns an error or matchAddr since it checks for equality prior to return. It's better to turn this into a function that just returns true if matchAddr was found in the list of addresses and false otherwise. In that case the return would be (bool, error). An alternative would be to return just error with nil signifying the address was found.