Add params for API v3 snapshot-create

In api-ref/source/v3/volumes-v3-snapshots.inc, the API for snapshot-create
is missing request params [metadata] and response params [updated_at], So
the patch is to add the missing param as well as the sample JSON files.

Change-Id: Id77525922e4b94613da35c4329c24f2fda8fda10

api-ref/source/v3/samples/snapshot-create-request.json

@@ -3,6 +3,7 @@
         "name": "snap-001",
         "description": "Daily backup",
         "volume_id": "5aa119a8-d25b-45a7-8d1b-88e127885635",
-        "force": true
+        "force": true,
+        "metadata": null
     }
 }

api-ref/source/v3/samples/snapshot-create-response.json

@@ -4,9 +4,11 @@
         "description": "Daily backup",
         "created_at": "2013-02-25T03:56:53.081642",
         "metadata": {},
+        "updated_at": "2013-02-25T03:58:53.081642",
         "volume_id": "5aa119a8-d25b-45a7-8d1b-88e127885635",
         "size": 1,
         "id": "ffa9bc5e-1172-4021-acaf-cdcd78a9584d",
         "name": "snap-001"
     }
 }
+

api-ref/source/v3/volumes-v3-snapshots.inc

@@ -97,6 +97,7 @@ Request
    - description: description
    - name: name
    - project_id: project_id_path
+   - metadata: metadata_7
 
 Request Example
 ---------------
@@ -118,6 +119,13 @@ Response Parameters
    - metadata: metadata
    - id: id
    - size: size
+   - updated_at: updated_at
+
+Response Example
+----------------
+
+.. literalinclude:: ./samples/snapshot-create-response.json
+   :language: javascript
 
 
 List accessible snapshots