Change some small compute clients to return one value and update tests

This includes availability_zone, certificates, extensions, instance_usage,
migrations, tenant_usages.

Partially implements: blueprint clients-return-one-value

Change-Id: I2a8ec52e177c865de1f159f556aa7adcf5ef0399
diff --git a/tempest/services/compute/json/migrations_client.py b/tempest/services/compute/json/migrations_client.py
index b41abc8..a65b655 100644
--- a/tempest/services/compute/json/migrations_client.py
+++ b/tempest/services/compute/json/migrations_client.py
@@ -31,4 +31,4 @@
         resp, body = self.get(url)
         body = json.loads(body)
         self.validate_response(schema.list_migrations, resp, body)
-        return resp, body['migrations']
+        return service_client.ResponseBodyList(resp, body['migrations'])