Automatic merge from submit-queue

Check existence of kubernetes dir for get-kube.sh

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There are a lot of references to https://get.k8s.io/ over the internet.
Most of such references do not describe KUBERNETES_SKIP_DOWNLOAD env variable
and newbies can get into a situation described below:

- execute `wget -q -O - https://get.k8s.io | bash`
- receive a failure due too missed packages or some configs
- fix the issue
- try again `wget -q -O - https://get.k8s.io | bash`

In this case, get-kube.sh will not check that kubernetes directory already
exist and repeat download again.
Lets make get-kube.sh more user-friendly and check existence of kubernetes dir