mdadm: /etc/mdadm/mdadm.conf defines no arrays.It turns out that this message is completely benign and you can ignore it, but that doesn't make it any less annoying right? Hopefully they get that worked out in Ubuntu 18.04 LTS next year. Until then, there is a quick way to fix it.
Just open /etc/mdadm/mdadm.conf with your favorite text editor and add the following lines at the bottom.
ARRAYSave the file, and that's it. No more stupid error.
If you didn't know, mdadm is the Linux software RAID software. If you are running Ubuntu in a VM though, chances are it is not going to have a software RAID to manage.
[H/T Ask Ubuntu]