acme/autocert: use ECDSA P-256 for account key and issued certs

ECDSA P-256 keys are smaller than RSA 2048 keys and signing
operations are faster.

Change-Id: I735f358f17ada4569de307a7af2934d4abf69b91
Reviewed-on: https://go-review.googlesource.com/28851
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>
1 file changed