Use backup-volfile-servers option for glusterfs mounts

First, glusterfs mount will use primary server, supplied by _param,
overridable on cluster-level.
If it fails, it will try to mount from each of 3 cluster nodes as a backup.

Change-Id: I5b7a68639263610a5e44f0865b13aa381ad9d127
diff --git a/glusterfs/client/volume/postgresql.yml b/glusterfs/client/volume/postgresql.yml
index d2337e7..087afa9 100644
--- a/glusterfs/client/volume/postgresql.yml
+++ b/glusterfs/client/volume/postgresql.yml
@@ -7,4 +7,4 @@
         postgresql:
           path: /srv/volumes/postgresql
           server: ${_param:postgresql_glusterfs_service_host}
-          opts: 'defaults'
+          opts: "defaults,backup-volfile-servers=${_param:cluster_node01_address}:${_param:cluster_node02_address}:${_param:cluster_node03_address}"