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use GetUniqueVolumeNameFromSpec instead of implementing it manually for kubelet volume manager

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

`volumeName` is only used for attachable plugin, so we should resolve it inside the `if` statement. Besides, we can use the already exist `GetUniqueVolumeNameFromSpec` mthod instead of invoking `GetVolumeName` and `GetUniqueVolumeName` manually.

**Release note**:

```release-note
NONE
```

/sig storage
/kind cleanup