"openssl ca" is keeping track of all generated certificates in its DB.
If we try to regenerate a certificate that was already generated,
openssl ca fails with :

ERROR:There is already a certificate for /C=XX/ST=(State,)/O=Company/CN=ca-shellinabox/emailAddress=xx@example.com

Changing "unique_subject" to no removes this ERROR.

/reviewed-on nexedi/slapos!556