One previously undocumented expectation is that
GetDynamicProvisionStorageClass can be called more than once per test
and then each time returns a new, unique storage class. The in-memory
implementation in driveroperations.go:GetStorageClass ensured that,
but loading from a .yaml file didn't. This caused the multivolume tests
to fail when applied to an already installed GCE driver with the
-storage.testdriver parameter.