Start with a clone of the ubuntu clean image.

Crete a user that will run the server proces:
useradd –comment “7days to die Server” -m 7days

change his shell:
chsh -s /bin/bash 7days

set his password:
passwd 7days

open the firewall for the ports of the server you decidet to run on:
firewall-cmd --permanent --add-port=26901/tcp
firewall-cmd --permanent --add-port=26901/udp
firewall-cmd --reload

Install prerequisites:
apt-get install wget
apt-get install screen
apt-get install lib32gcc1

get the Steam client:
cd /home/7days
wget http://media.steampowered.com/installer/steamcmd_linux.tar.gz

unpack it and cleanup:
tar xvzf steamcmd_linux.tar.gz
rm steamcmd_linux.tar.gz

login to staem and get the 7days to die server software:
./steamcmd.sh

login yoursteamusername
password yoursteampassword
force_install_dir ./7daysded
app_update 294420 
quit