acme service: generate a CA for self-signed certificate

This is needed because simp_le expects two certificates in fullchain.pem, leading to error:

> Not enough PEM encoded messages were found in fullchain.pem; at least 2 were expected, found 1.

We now create a CA and sign the key with it instead, providing correct fullchain.pem.

Also cleanup service a bit -- use PATH and a private temporary directory (which
is more suitable).