make 'server' as 'required' in server update schema

This patch make the 'server' attribute as 'required' in
server update schema (base_update_server).

This attribute is not optional in server's update API response.

Partially implements blueprint nova-api-attribute-test

Change-Id: Ibec518910597fc9e03a69d1d2bc94e719cfab0f9
diff --git a/tempest/api_schema/compute/servers.py b/tempest/api_schema/compute/servers.py
index 4e8c201..33992b1 100644
--- a/tempest/api_schema/compute/servers.py
+++ b/tempest/api_schema/compute/servers.py
@@ -92,7 +92,8 @@
                              'user_id', 'tenant_id', 'created', 'updated',
                              'metadata', 'links', 'addresses']
             }
-        }
+        },
+        'required': ['server']
     }
 }