Check reserve/unreserve fixed-ips APIs attributes

This patch adds the JSON schema for Nova V2 reserve & unreserve
fixed-ips APIs response and validate the response with added JSON
schema to block the backward incompatibility change in the future.

The response body of V2 reserve & unreserve fixed-ip API is below:

{
    "202 Accepted The request is accepted for processing."
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ic837fb3cdc34a79c25589a7263f54b96b6043680
diff --git a/tempest/api_schema/compute/v2/fixed_ips.py b/tempest/api_schema/compute/v2/fixed_ips.py
index a6add04..446633f 100644
--- a/tempest/api_schema/compute/v2/fixed_ips.py
+++ b/tempest/api_schema/compute/v2/fixed_ips.py
@@ -34,3 +34,8 @@
         'required': ['fixed_ip']
     }
 }
+
+fixed_ip_action = {
+    'status_code': [202],
+    'response_body': {'type': 'string'}
+}
diff --git a/tempest/services/compute/json/fixed_ips_client.py b/tempest/services/compute/json/fixed_ips_client.py
index 5fdd564..f2d5cbe 100644
--- a/tempest/services/compute/json/fixed_ips_client.py
+++ b/tempest/services/compute/json/fixed_ips_client.py
@@ -39,4 +39,5 @@
         """This reserves and unreserves fixed ips."""
         url = "os-fixed-ips/%s/action" % (ip)
         resp, body = self.post(url, json.dumps(body))
+        self.validate_response(schema.fixed_ip_action, resp, body)
         return resp, body