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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