FAI 5.6 and Ubuntu 18.04 LTS bionic

[Thomas Lange]

After building the basefile for Ubuntu 18.04 LTS aka Bionic, my first
tests look fine. Setting the release name in class/UBUNTU.var to
ubuntudist=bionic
and copying the basefile from
https://fai-project.org/download/basefiles/BIONIC64.tar.xz
works. Oh, I didn't test yet if the network is running after the
installation. I will do this later.

--
regards Thomas