pki: parse keys from certificates into ssh_key
Previously, we were just storing the raw certificate in ssh_key->cert. With
this change, we parse out the nonce string at the beginning of the certificate.
This leaves us with the certificate's public key at the start of the buffer.
The existing public key parsing code can then parse this out into the key fieds
of the ssh_key.
Signed-off-by: Ben Toews <email@example.com>
Reviewed-by: Jakub Jelen <firstname.lastname@example.org>