MINOR: ssl: Set default dh size to 2048

MINOR: ssl: Set default dh size to 2048

Starting from OpenSSLv3, we won't rely on the
SSL_CTX_set_tmp_dh_callback mechanism so we will need to know the DH
size we want to use during init. In order for the default DH param size
to be used when no RSA or DSA private key can be found for a given bind
line, we will need to know the default size we want to use (which was
not possible the way the code was built, since the global default dh
size was set too late.