Automatic merge from submit-queue (batch tested with PRs 64308, 64367, 64165, 64274). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubeadm: rename the `kube-dns` phases addon

**What this PR does / why we need it**:
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.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes https://github.com/kubernetes/kubeadm/issues/856

**Special notes for your reviewer**:
Please verify if we are OK with the "hard rename".

**Release note**:

```release-note
kubeadm: rename the addon parameter `kube-dns` to `coredns` for `kubeadm alpha phases addons` as CoreDNS is now the default DNS server in 1.11.
```

@kubernetes/sig-cluster-lifecycle-pr-reviews 
/area kubeadm