Automatic merge from submit-queue
Reserve kubernetes.io and k8s.io namespace for flex volume options
Split from https://github.com/kubernetes/kubernetes/pull/39488.
Flex volume already stuffs system information into the options map, and assumes it is free to do so:
```
optionFSType = "kubernetes.io/fsType"
optionReadWrite = "kubernetes.io/readwrite"
optionKeySecret = "kubernetes.io/secret"
```
this formalizes that by reserving the `kubernetes.io` and `k8s.io` namespaces so that user-specified options are never stomped by the system, and flex plugins can know that options with those namespaces came from the system, not user-options.
```release-note
Parameter keys in a StorageClass `parameters` map may not use the `kubernetes.io` or `k8s.io` namespaces.
```
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