Change root CA key size to 256 bits

[aaronlehmann]

Our current root CA key generation creates P384 ECDSA keys. It turns out that Go's P384 implementaiton is very slow. Doing a TLS handshake with keys signed by this root CA key takes 100 ms on a modern laptop, and nearly 2 seconds on a slow ARM machine - see #1364. This could potentially cause problems when many nodes reconnect at the same time, say after a network partition is resolved.

I think we should change the root CA key size to 256 bits, to take advantage of P256 optimizations. I think this will provide a sufficient security level. In my benchmarks, the handshakes go down to about 3 ms on my laptop.

cc @tonistiigi @diogomonica @NathanMcCauley