Implement cluster maintenance mode

Implements a "maintenance mode" for PVC clusters. For now, the only
thing this mode does is disable node fencing while the state is true.
This allows the administrator to tell PVC that network connectivity,
etc. might be interrupted and to avoid fencing nodes.

Closes #70

docs/manuals/swagger.json

@@ -3653,6 +3653,36 @@
                 "tags": [
                     "root"
                 ]
+            },
+            "post": {
+                "description": "",
+                "parameters": [
+                    {
+                        "description": "The cluster maintenance state",
+                        "in": "query",
+                        "name": "state",
+                        "required": true,
+                        "type": "boolean"
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/Message"
+                        }
+                    },
+                    "400": {
+                        "description": "Bad request",
+                        "schema": {
+                            "$ref": "#/definitions/Message"
+                        }
+                    }
+                },
+                "summary": "Set the cluster maintenance mode",
+                "tags": [
+                    "node"
+                ]
             }
         },
         "/api/v1/storage/ceph/option": {