MINOR: ssl: Add support for EC for the CA used to sign generated certificates

MINOR: ssl: Add support for EC for the CA used to sign generated certificates

This is done by adding EVP_PKEY_EC type in supported types for the CA private
key when we get the message digest used to sign a generated X509 certificate.
So now, we support DSA, RSA and EC private keys.

And to be sure, when the type of the private key is not directly supported, we
get its default message digest using the function
'EVP_PKEY_get_default_digest_nid'.

We also use the key of the default certificate instead of generated it. So we
are sure to use the same key type instead of always using a RSA key.