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Compare correct file names for volume detach operation

**What this PR does / why we need it**:
Current volume detach code compares volume path with disk path, as it is. This PR removes '.vmdk' extension from both paths and then compares them. This makes sure that correct comparison is done irrespective of a missing '.vmdk' extension from one of the paths.

**Which issue(s) this PR fixes**:
Fixes  https://github.com/vmware/kubernetes/issues/392 

**Special notes for your reviewer**:
Deployed cluster on vSphere and provisioned a static volume. Verified that a statically provisioned volume gets detached even when volume path didn't contain any .vmdk extension and disk path had .vmdk extension.

**Release note**:
```vSphere cloud provider: Fix detach operation for volumes, when .vmdk extension is not specified in volume path.```