Rearrange certificates response schema into one file

After removing v3 schemas, we have only 1 set of schemas for v2 (/v2.1)
APIs but those end up in scattered structure.

Schema files needs to be re arranged into a clean structure. Any
resource schema should be defined in single file for better readability.

This patch merge the certificates response schema into one file.

Partially implements blueprint rearrange-nova-response-schemas

Change-Id: I50752ca74f08029c16913bea98361d84d04aa90c
diff --git a/tempest/services/compute/json/certificates_client.py b/tempest/services/compute/json/certificates_client.py
index 43ec917..e6b72bb 100644
--- a/tempest/services/compute/json/certificates_client.py
+++ b/tempest/services/compute/json/certificates_client.py
@@ -15,8 +15,7 @@
 
 import json
 
-from tempest.api_schema.response.compute import certificates as schema
-from tempest.api_schema.response.compute.v2_1 import certificates as v2schema
+from tempest.api_schema.response.compute.v2_1 import certificates as schema
 from tempest.common import service_client
 
 
@@ -34,5 +33,5 @@
         url = "os-certificates"
         resp, body = self.post(url, None)
         body = json.loads(body)
-        self.validate_response(v2schema.create_certificate, resp, body)
+        self.validate_response(schema.create_certificate, resp, body)
         return service_client.ResponseBody(resp, body['certificate'])