util: add CSIDriver.GetInstanceID()

There has been some confusion about using different variables for the
InstanceID of the RBD-driver. By removing the global variable
CSIInstanceID, there should be no confusion anymore what variable to
use.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

internal/controller/controller.go

@@ -39,6 +39,7 @@ type Config struct {
 	DriverName  string
 	Namespace   string
 	ClusterName string
+	InstanceID  string
 	SetMetadata bool
 }