Add enableHttp2 to server.yml template for all frameworks

[stevehu]

Http2 is supported on the server side for a long time but the client is still working in progress. To enable Http2 on server side, the user has to added enableHttp2: true in server.yml manually. Now we are adding this into the server.yml template so that users can specify if they want http2 enabled in the generator config.json.

[stevehu]

Currently, http2 is enabled by default in the server.yml template. Will re-evaluate later if we need to make it configurable. So far I don't see any negative impact to enable it by default.