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/gerrit.yml b/glusterfs/client/volume/gerrit.yml
index 6199aa6..39e9559 100644
--- a/glusterfs/client/volume/gerrit.yml
+++ b/glusterfs/client/volume/gerrit.yml
@@ -7,6 +7,6 @@
         gerrit:
           path: /srv/volumes/gerrit
           server: ${_param:gerrit_glusterfs_service_host}
-          opts: 'defaults'
+          opts: "defaults,backup-volfile-servers=${_param:cluster_node01_address}:${_param:cluster_node02_address}:${_param:cluster_node03_address}"
           user: 1000
           group: 1000