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
```
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