Update retrieve-cluster-backup JSON output schema

Define the "single_node_cluster" attribute.

core/imageroot/var/lib/nethserver/cluster/actions/retrieve-cluster-backup/validate-output.json

@@ -5,6 +5,7 @@
     "description": "Return info from cluster backup",
     "examples": [
         {
+            "single_node_cluster": true,
             "cluster": "mycluster1",
             "vpn": "cs1.leader.cluster0.gs.nethserver.net:55820",
             "domains": 1,
@@ -13,12 +14,17 @@
     ],
     "type": "object",
     "required": [
+        "single_node_cluster",
         "cluster",
         "vpn",
         "domains",
         "backup_repositories"
     ],
     "properties": {
+        "single_node_cluster": {
+            "type": "boolean",
+            "description": "If true, the backup refers to a cluster of a single node"
+        },
         "cluster": {
             "title": "Cluster label",
             "description": "User defined cluster label, can be empty'",
@@ -40,4 +46,4 @@
             "type": "integer"
         }
     }
-}
+}