Create a script to implement account key rollover

ACME RFC allows users to rollover their private account key.

It will be useful to have a little script to apply this modification to avoid using compromised account private key or to allow to upgrade RSA keys to ECDSA for example (or even modifies number of bytes used in these keys).

I suggest to do a little script like we have done for account deletion (which will be deactivation in future).

I think this script won't need a configuration file, but just two arguments to indicate old account private key and new private key. That's better, because we won't need code to read configuration file and because this script should be a one shot script which certainly won't be used in a cron job.