Use volume ID as a prefix if the bucket is fixed in the storage class

With this, each volume will get its own prefix within the bucket if it
is configured in the storage class. This also ensures backwards
compatibility with older volumes that have been created in earlier
versions of csi-s3.

deploy/kubernetes/examples/pod.yaml

@@ -0,0 +1,17 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: csi-s3-test-nginx
+  namespace: default
+spec:
+  containers:
+   - name: csi-s3-test-nginx
+     image: nginx
+     volumeMounts:
+       - mountPath: /var/lib/www/html
+         name: webroot
+  volumes:
+   - name: webroot
+     persistentVolumeClaim:
+       claimName: csi-s3-pvc
+       readOnly: false