The command `kubeadm alpha phases addon` has a property
called `kube-dns` which would install kube-dns, pre 1.11.

In the case of 1.11 this property will install CoreDNS,
because the property is also bound to the `CoreDNS` feature gate,
which is now `true` by default.

Fix that by renaming the property to `coredns`, updating the Cobra
info and also updating the unit tests.