Automatic merge from submit-queue (batch tested with PRs 56650, 55813, 56911, 56921, 56871). 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>.

enable flexvolume on Windows node

**What this PR does / why we need it**:
enable flexvolume on Windows node:
current PodVolumeDir is like 
`\var\lib\kubelet\pods\f54c5a74-da63-11e7-b71a-000d3a02c330\volumes\test~hostpath.cmd\flextest`
which is a unix path, with this PR, PodVolumeDir in **windows** kubelet  will be converted into like
`c:\var\lib\kubelet\pods\f54c5a74-da63-11e7-b71a-000d3a02c330\volumes\test~hostpath.cmd\flextest` 

**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 #56875

**Special notes for your reviewer**:
Detailed steps about how to use flexvolume on Windows with this PR:
https://github.com/andyzhangx/Demo/tree/master/windows/flexvolume

**Release note**:

```
enable flexvolume on Windows node
```

/sig windows