The official vagrant box of Ubuntu Xenial 64 was initially with a issue: it didn’t have a vagrant user (according to Vagrant recommendations all boxes should have a user with name vagrant and password vagrant).
A recent release fixed the problem in a partial way: the user vagrant was added, but it is not possible to use it with password authentication (a feature that I usually need in my workshops).
So to fix this issue I created a new vagrant box based on the official Xenial box. I added the following lines in the shell provisioner:
echo "vagrant:vagrant" | sudo chpasswd sed -i -e 's/PasswordAuthentication no/PasswordAuthentication yes/' /etc/ssh/sshd_config
The first line set the password vagrant for the user vagrant. The second line enables PasswordAuthentication.