Fix #95: Call GetSnapshotStatus to check snapshot status

[cwdsuzhou]

Fix #95.
In csi-snapshotter, function "GetSnapshotStatus" is never called and "CreateSnapshot" is called in function "GetSnapshotStatus".

In this PR, I have fixed #95 and changed the some unit tests.

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[xing-yang]

We replaced the call to ListSnapshots with CreateSnapshot to check snapshot status when adding support to CSI spec 1.0:
#61

As stated in the CSI spec, ListSnapshots is optional. CreateSnapshot is the preferred way to check snapshot status.
https://github.com/container-storage-interface/spec/blob/master/spec.md#createsnapshot-deletesnapshot-listsnapshots

The CreateSnapshot call in a CSI driver should be idempotent. So repeated call to CreateSnapshot with same parameters should return the same result and just create one snapshot, not multiples.