Remove host option

Update cinder backend config for ceph,
backend parameter "host" has been deprecated starting from Kilo,
and changed to "backend_host", and, since Pike, it was removed
completely.

Change-Id: I7c20d6cfd32fa37c9bfe33d0938f71e5afb73c64
Related-Prod: PROD-22192 (PROD:22192)
diff --git a/README.rst b/README.rst
index 555ef91..b7427aa 100644
--- a/README.rst
+++ b/README.rst
@@ -440,6 +440,7 @@
             ceph_backend:
               type_name: standard-iops
               backend: ceph_backend
+              backend_host: ceph
               pool: volumes
               engine: ceph
               user: cinder