OK, I think I'm confused.
Starting back up at the top:
I was setting up the web server, certificates etc.. and when I rebooted and try to acces the localhost I get the following.
"It works!
This is the default web page for this server.
The web server software is running but no content has been added, yet."
That suggests the possibility of an attempt to access Zentyal from a second machine, not an attempt to access Zentyal as localhost (since Firefox automatically opens to the correct address, normally). This is what I would normally see if I went to
http://myserverip/ and had added no content for the Zentyal web server module to dispaly.
If that's the case, you need to type in not
https://localhost/ on that second machine, but
https://yourserverip/. So, for example, if your server's local address was 192.168.1.1, you would type
https://192.168.1.1/ to access the Zentyal web interface.
Accessing the Zentyal machine directly, with an attached mouse, keyboard, and display, accesses not the web interface but the Zentyal desktop, which can give you access to the web interface via localhost.
I'm probably just misunderstanding something, but in no case that I'm aware of should
https://localhost/ give you the "It works!" message.