Implements pillar key defining volume backend driver (volume_driver)

Reclass model example to set driver class:

cinder:
  controller:
    backend:
      FJISCSI:
        driver: cinder.volume.drivers.fujitsu.eternus_dx.eternus_dx_iscsi.FJDXISCSIDriver
        engine: fujitsu

Change-Id: Idc0bc5c49ba08161333742d8c3bb171023eb5415
Closes-Bug: PROD-30853
Related-Prod: PROD-30853
diff --git a/README.rst b/README.rst
index 5b55e69..44ee5f1 100644
--- a/README.rst
+++ b/README.rst
@@ -518,6 +518,20 @@
             connection: FC/iSCSI
             name: 10k_SAS
 
+* Cinder setup with Fujitsu Eternus. Set driver class to be used by cinder-volume:
+  .. code-block:: yaml
+
+    cinder:
+      controller:
+        enabled: True
+        backend:
+          FJISCSI:
+            driver: cinder.volume.drivers.fujitsu.eternus_dx.eternus_dx_iscsi.FJDXISCSIDriver
+            engine: fujitsu
+          FJFC:
+            driver: cinder.volume.drivers.fujitsu.eternus_dx.eternus_dx_fc.FJDXFCDriver
+            engine: fujitsu
+
 * Cinder setup with IBM GPFS filesystem:
 
   .. code-block:: yaml