Automatic merge from submit-queue

kubeadm: improve quickstart instructions

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

Improves instructional output following setup of a kubernetes master with kubeadm.

This helps prevent unnecessary support overhead such as: https://github.com/kubernetes/kubeadm/issues/212#issuecomment-292015402

**Example current output**:

```
To start using your cluster, you need to run (as a regular user):

  sudo cp /etc/kubernetes/admin.conf $HOME/
  sudo chown $(id -u):$(id -g) $HOME/admin.conf
  export KUBECONFIG=$HOME/admin.conf
```

**Example new output**:

```
To start using your cluster, you need to run (as a regular user):

  mkdir -p $HOME/.kube
  sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
  sudo chown $(id -u):$(id -g) $HOME/.kube/config
```

**Release note**:

```release-note
NONE
```