I did this with a new 1604 but zentyal-samba is broken since weeks. Also no update since that time. I think the apt/soure.list entry points to a dead repository.
Nope its there, commit updates 2 days ago, be patient
and I just installed again, repos good, as i posted in another thread:
"zentyal-squid, zentyal-openvpn, zentyal-mail, zentyal-dhcp, and zentyal-printers are the modules needed to complete the package."