MINOR: acme: generate a temporary key pair

MINOR: acme: generate a temporary key pair

This patch provides two functions acme_gen_tmp_pkey() and
acme_gen_tmp_x509().

These functions generates a unique keypair and X509 certificate that
will be stored in tmp_x509 and tmp_pkey. If the key pair or certificate
was already generated they will return the existing one.

The key is an RSA2048 and the X509 is generated with a expiration in the
past. The CN is "expired".

These are just placeholders to be used if we don't have files.