Implement mounting via stage directory

Previously, multiple containers with the same mounted volume resulted in multiple
FUSE processes. This behaviour was breaking parallel modifications from different
containers, consumed extra resources, and after mounting via systemd was introduced,
led to the total inability to mount the same volume into multiple containers on
the same host.

Now only one FUSE process is started per volume, per host.

deploy/kubernetes/csi-s3.yaml

@@ -103,6 +103,9 @@ spec:
           volumeMounts:
             - name: plugin-dir
               mountPath: /csi
+            - name: stage-dir
+              mountPath: /var/lib/kubelet/plugins/kubernetes.io/csi/ru.yandex.s3.csi
+              mountPropagation: "Bidirectional"
             - name: pods-mount-dir
               mountPath: /var/lib/kubelet/pods
               mountPropagation: "Bidirectional"
@@ -119,6 +122,10 @@ spec:
           hostPath:
             path: /var/lib/kubelet/plugins/ru.yandex.s3.csi
             type: DirectoryOrCreate
+        - name: stage-dir
+          hostPath:
+            path: /var/lib/kubelet/plugins/kubernetes.io/csi/ru.yandex.s3.csi
+            type: DirectoryOrCreate
         - name: pods-mount-dir
           hostPath:
             path: /var/lib/kubelet/pods