MEDIUM: ssl: Add certificate password callback that calls external command

MEDIUM: ssl: Add certificate password callback that calls external command

When a certificate is protected by a password, we can provide the
password via the dedicated pem_password_cb param provided to
PEM_read_bio_PrivateKey.
HAProxy will fetch the password automatically during init by calling a
user-defined external command that should dump the right password on its
standard output (see new 'ssl-passphrase-cmd' global option).