Current and N-1 versions of the spec adopted TLS-SNI-02, but Let's Encrypt seems to support TLS-SNI-01, devised in v1 of the spec. This change implements both, older 01 and current 02 versions of TLS-SNI challenges. Change-Id: I2bb76ad852e105098f703cb6113d3996c0e0b118 Reviewed-on: https://go-review.googlesource.com/25450 Reviewed-by: Alex Vaghin <ddos@google.com>