Check attributes of image meta item Nova APIs

This patch adds the JSON Schema for Nova V2 Image meta item APIs
(set & get meta item) and validate the response with added JSON
Schema to block the backward incompatibility change in the future.

Response body of above APIs is below-

{
    "meta":
    {

    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ic00fc3b4750057b641ef01738e34f7242e27bb1d
diff --git a/tempest/api_schema/compute/v2/images.py b/tempest/api_schema/compute/v2/images.py
index 7fd6b84..fad6b56 100644
--- a/tempest/api_schema/compute/v2/images.py
+++ b/tempest/api_schema/compute/v2/images.py
@@ -109,3 +109,14 @@
         'required': ['metadata']
     }
 }
+
+image_meta_item = {
+    'status_code': [200],
+    'response_body': {
+        'type': 'object',
+        'properties': {
+            'meta': {'type': 'object'}
+        },
+        'required': ['meta']
+    }
+}