connection to https backend may need Server Name Indication (SNI)

[jmuk]

When connecting to the backend https server, sometimes the backend server may need the Server Name Indication (SNI) to choose the proper certificate.

In nginx, it needs proxy_ssl_name and proxy_ssl_server_name on; configurations (see http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_name and http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_server_name). We may probably want to specify those configs optionally on https connection.