lib/cinder: add sheepdog support

This enables us to use Sheepdog as a Cinder backend storage by setting
the CINDER_DRIVER environment variable.

Change-Id: I70cfb7f89ac3260d277fa160c457d220255de065
diff --git a/lib/cinder b/lib/cinder
index 8b1ccd7..a730cd6 100644
--- a/lib/cinder
+++ b/lib/cinder
@@ -209,6 +209,8 @@
             iniset $CINDER_CONF DEFAULT xenapi_nfs_server "$CINDER_XENAPI_NFS_SERVER"
             iniset $CINDER_CONF DEFAULT xenapi_nfs_serverpath "$CINDER_XENAPI_NFS_SERVERPATH"
         )
+    elif [ "$CINDER_DRIVER" == "sheepdog" ]; then
+        iniset $CINDER_CONF DEFAULT volume_driver "cinder.volume.drivers.sheepdog.SheepdogDriver"
     fi
 }