Replying to myself.
I found the following issue - my question was duplicate to that issue, so please ignore mine.
It seems a command line API is not available for SSL update for AppEngine.
Support automatic certificate request via Lets Encrypt/ACME
https://code.google.com/p/googleappengine/issues/detail?id=12535>Jul 27, 2016 All we need is a gcloud command to update the app engine SSL cert, and then we can use whatever certification authority we want to whether it's LE, or even StartEncrypt is going to start using ACME soon.
Thanks,
Richard
2016年8月6日土曜日 18時50分59秒 UTC+9 Richard Imaoka: