tests/system: adapt to new API response

In elastic/kibana#95146
the response structure for listing APM agent central
config changed. Update system tests to match.

tests/system/kibana.py

@@ -61,7 +61,7 @@ def list_agent_config(self):
             }
         )
         assert resp.status_code == 200, resp.status_code
-        return resp.json()
+        return resp.json()['configurations']
 
     def delete_agent_config(self, service, env=None):
         data = {