You don't need such "trusted" certificate
There is something not well understood about certificates nowadays: trusting certificate is only matter of... trust
I'm not joking: if you trust certificate exposed by server you access (here, e.g. your Zentyal server), you can easily add Zentyal CA in the list of trusted CA and you will jot be warned anymore.
Reason why you are not warned with most of the public CAs is that editors like Microsoft, Mozilla etc... include these CA in the list of trusted CA. This doesn't prevent to buy certificate from public CA company that is not in the list of default trusted CA.
So for personal use only, save some money and use your own CA that you trust