Ubuntu 18.04 LTS server full disk encryption

According to the 18.04 server release notes on

https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes#Ubuntu_Server

you will need to use the alternate installer to configure LVM (also RAID, multipath, vlans, bonds, or the ability to re-using existing partitions) on installation. You can download it from:

http://cdimage.ubuntu.com/releases/18.04/release/